Tree states optimization using EpochCache (#4429)

* Relocate epoch cache to BeaconState

* Optimize per block processing by pulling previous epoch & current epoch calculation up.

* Revert `get_cow` change (no performance improvement)

* Initialize `EpochCache` in epoch processing and load it from state when getting base rewards.

* Initialize `EpochCache` at start of block processing if required.

* Initialize `EpochCache` in `transition_blocks` if `exclude_cache_builds` is enabled

* Fix epoch cache initialization logic

* Remove FIXME comment.

* Cache previous & current epochs in `consensus_context.rs`.

* Move `get_base_rewards` from `ConsensusContext` to `BeaconState`.

* Update Milhouse version

consensus/state_processing/src/per_epoch_processing/altair.rs

@@ -1,4 +1,5 @@
 use super::{process_registry_updates, process_slashings, EpochProcessingSummary, Error};
+use crate::epoch_cache::initialize_epoch_cache;
 use crate::per_epoch_processing::{
     effective_balance_updates::process_effective_balance_updates,
     historical_roots_update::process_historical_roots_update,
@@ -75,6 +76,7 @@ pub fn process_epoch<T: EthSpec>(
 
     // Rotate the epoch caches to suit the epoch transition.
     state.advance_caches(spec)?;
+    initialize_epoch_cache(state, state.next_epoch()?, spec)?;
 
     Ok(EpochProcessingSummary::Altair {
         participation_cache,