Cache participating indices for Altair epoch processing (#2416)

## Issue Addressed

NA

## Proposed Changes

This PR addresses two things:

1. Allows the `ValidatorMonitor` to work with Altair states.
1. Optimizes `altair::process_epoch` (see [code](https://github.com/paulhauner/lighthouse/blob/participation-cache/consensus/state_processing/src/per_epoch_processing/altair/participation_cache.rs) for description)

## Breaking Changes

The breaking changes in this PR revolve around one premise:

*After the Altair fork, it's not longer possible (given only a `BeaconState`) to identify if a validator had *any* attestation included during some epoch. The best we can do is see if that validator made the "timely" source/target/head flags.*

Whilst this seems annoying, it's not actually too bad. Finalization is based upon "timely target" attestations, so that's really the most important thing. Although there's *some* value in knowing if a validator had *any* attestation included, it's far more important to know about "timely target" participation, since this is what affects finality and justification.

For simplicity and consistency, I've also removed the ability to determine if *any* attestation was included from metrics and API endpoints. Now, all Altair and non-Altair states will simply report on the head/target attestations.

The following section details where we've removed fields and provides replacement values.

### Breaking Changes: Prometheus Metrics

Some participation metrics have been removed and replaced. Some were removed since they are no longer relevant to Altair (e.g., total attesting balance) and others replaced with gwei values instead of pre-computed values. This provides more flexibility at display-time (e.g., Grafana).

The following metrics were added as replacements:

- `beacon_participation_prev_epoch_head_attesting_gwei_total`
- `beacon_participation_prev_epoch_target_attesting_gwei_total`
- `beacon_participation_prev_epoch_source_attesting_gwei_total`
- `beacon_participation_prev_epoch_active_gwei_total`

The following metrics were removed:

- `beacon_participation_prev_epoch_attester`
   - instead use `beacon_participation_prev_epoch_source_attesting_gwei_total / beacon_participation_prev_epoch_active_gwei_total`.
- `beacon_participation_prev_epoch_target_attester`
   - instead use `beacon_participation_prev_epoch_target_attesting_gwei_total / beacon_participation_prev_epoch_active_gwei_total`.
- `beacon_participation_prev_epoch_head_attester`
   - instead use `beacon_participation_prev_epoch_head_attesting_gwei_total / beacon_participation_prev_epoch_active_gwei_total`.

The `beacon_participation_prev_epoch_attester` endpoint has been removed. Users should instead use the pre-existing `beacon_participation_prev_epoch_target_attester`. 

### Breaking Changes: HTTP API

The `/lighthouse/validator_inclusion/{epoch}/{validator_id}` endpoint loses the following fields:

- `current_epoch_attesting_gwei` (use `current_epoch_target_attesting_gwei` instead)
- `previous_epoch_attesting_gwei` (use `previous_epoch_target_attesting_gwei` instead)

The `/lighthouse/validator_inclusion/{epoch}/{validator_id}` endpoint lose the following fields:

- `is_current_epoch_attester` (use `is_current_epoch_target_attester` instead)
- `is_previous_epoch_attester` (use `is_previous_epoch_target_attester` instead)
- `is_active_in_current_epoch` becomes `is_active_unslashed_in_current_epoch`.
- `is_active_in_previous_epoch` becomes `is_active_unslashed_in_previous_epoch`.

## Additional Info

NA

## TODO

- [x] Deal with total balances
- [x] Update validator_inclusion API
- [ ] Ensure `beacon_participation_prev_epoch_target_attester` and `beacon_participation_prev_epoch_head_attester` work before Altair

Co-authored-by: realbigsean <seananderson33@gmail.com>

consensus/state_processing/src/per_epoch_processing/altair.rs

@@ -3,10 +3,10 @@ use crate::per_epoch_processing::{
     effective_balance_updates::process_effective_balance_updates,
     historical_roots_update::process_historical_roots_update,
     resets::{process_eth1_data_reset, process_randao_mixes_reset, process_slashings_reset},
-    validator_statuses::ValidatorStatuses,
 };
 pub use inactivity_updates::process_inactivity_updates;
 pub use justification_and_finalization::process_justification_and_finalization;
+pub use participation_cache::ParticipationCache;
 pub use participation_flag_updates::process_participation_flag_updates;
 pub use rewards_and_penalties::process_rewards_and_penalties;
 pub use sync_committee_updates::process_sync_committee_updates;
@@ -14,6 +14,7 @@ use types::{BeaconState, ChainSpec, EthSpec, RelativeEpoch};
 
 pub mod inactivity_updates;
 pub mod justification_and_finalization;
+pub mod participation_cache;
 pub mod participation_flag_updates;
 pub mod rewards_and_penalties;
 pub mod sync_committee_updates;
@@ -27,13 +28,16 @@ pub fn process_epoch<T: EthSpec>(
     state.build_committee_cache(RelativeEpoch::Current, spec)?;
     state.build_committee_cache(RelativeEpoch::Next, spec)?;
 
-    // Justification and finalization.
-    process_justification_and_finalization(state, spec)?;
+    // Pre-compute participating indices and total balances.
+    let participation_cache = ParticipationCache::new(state, spec)?;
 
-    process_inactivity_updates(state, spec)?;
+    // Justification and finalization.
+    process_justification_and_finalization(state, &participation_cache)?;
+
+    process_inactivity_updates(state, &participation_cache, spec)?;
 
     // Rewards and Penalties.
-    process_rewards_and_penalties(state, spec)?;
+    process_rewards_and_penalties(state, &participation_cache, spec)?;
 
     // Registry Updates.
     process_registry_updates(state, spec)?;
@@ -41,7 +45,7 @@ pub fn process_epoch<T: EthSpec>(
     // Slashings.
     process_slashings(
         state,
-        state.get_total_active_balance(spec)?,
+        participation_cache.current_epoch_total_active_balance(),
         spec.proportional_slashing_multiplier_altair,
         spec,
     )?;
@@ -69,14 +73,7 @@ pub fn process_epoch<T: EthSpec>(
     // Rotate the epoch caches to suit the epoch transition.
     state.advance_caches()?;
 
-    // FIXME(altair): this is an incorrect dummy value, we should think harder
-    // about how we want to unify validator statuses between phase0 & altair.
-    // We should benchmark the new state transition and work out whether Altair could
-    // be accelerated by some similar cache.
-    let validator_statuses = ValidatorStatuses::new(state, spec)?;
-
-    Ok(EpochProcessingSummary {
-        total_balances: validator_statuses.total_balances,
-        statuses: validator_statuses.statuses,
+    Ok(EpochProcessingSummary::Altair {
+        participation_cache,
     })
 }