Optimize validator duties (#2243)

## Issue Addressed

Closes #2052

## Proposed Changes

- Refactor the attester/proposer duties endpoints in the BN
    - Performance improvements
    - Fixes some potential inconsistencies with the dependent root fields.
    - Removes `http_api::beacon_proposer_cache` and just uses the one on the `BeaconChain` instead.
    - Move the code for the proposer/attester duties endpoints into separate files, for readability.
- Refactor the `DutiesService` in the VC
    - Required to reduce the delay on broadcasting new blocks.
    - Gets rid of the `ValidatorDuty` shim struct that came about when we adopted the standard API.
    - Separate block/attestation duty tasks so that they don't block each other when one is slow.
- In the VC, use `PublicKeyBytes` to represent validators instead of `PublicKey`. `PublicKey` is a legit crypto object whilst `PublicKeyBytes` is just a byte-array, it's much faster to clone/hash `PublicKeyBytes` and this change has had a significant impact on runtimes.
    - Unfortunately this has created lots of dust changes.
 - In the BN, store `PublicKeyBytes` in the `beacon_proposer_cache` and allow access to them. The HTTP API always sends `PublicKeyBytes` over the wire and the conversion from `PublicKey` -> `PublickeyBytes` is non-trivial, especially when queries have 100s/1000s of validators (like Pyrmont).
 - Add the `state_processing::state_advance` mod which dedups a lot of the "apply `n` skip slots to the state" code.
    - This also fixes a bug with some functions which were failing to include a state root as per [this comment](072695284f/consensus/state_processing/src/state_advance.rs (L69-L74)). I couldn't find any instance of this bug that resulted in anything more severe than keying a shuffling cache by the wrong block root.
 - Swap the VC block service to use `mpsc` from `tokio` instead of `futures`. This is consistent with the rest of the code base.
    
~~This PR *reduces* the size of the codebase 🎉~~ It *used* to reduce the size of the code base before I added more comments. 

## Observations on Prymont

- Proposer duties times down from peaks of 450ms to consistent <1ms.
- Current epoch attester duties times down from >1s peaks to a consistent 20-30ms.
- Block production down from +600ms to 100-200ms.

## Additional Info

- ~~Blocked on #2241~~
- ~~Blocked on #2234~~

## TODO

- [x] ~~Refactor this into some smaller PRs?~~ Leaving this as-is for now.
- [x] Address `per_slot_processing` roots.
- [x] Investigate slow next epoch times. Not getting added to cache on block processing?
- [x] Consider [this](072695284f/beacon_node/store/src/hot_cold_store.rs (L811-L812)) in the scenario of replacing the state roots


Co-authored-by: pawan <pawandhananjay@gmail.com>
Co-authored-by: Michael Sproul <michael@sigmaprime.io>

account_manager/src/validator/create.rs

@@ -229,7 +229,7 @@ pub fn cli_run<T: EthSpec>(
         })?;
 
         slashing_protection
-            .register_validator(&voting_pubkey)
+            .register_validator(voting_pubkey.compress())
             .map_err(|e| {
                 format!(
                     "Error registering validator {}: {:?}",

account_manager/src/validator/import.rs

@@ -236,7 +236,7 @@ pub fn cli_run(matches: &ArgMatches, validator_dir: PathBuf) -> Result<(), Strin
             .public_key()
             .ok_or_else(|| format!("Keystore public key is invalid: {}", keystore.pubkey()))?;
         slashing_protection
-            .register_validator(&voting_pubkey)
+            .register_validator(voting_pubkey.compress())
             .map_err(|e| {
                 format!(
                     "Error registering validator {}: {:?}",