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>

beacon_node/beacon_chain/src/snapshot_cache.rs

@@ -14,14 +14,18 @@ pub struct PreProcessingSnapshot<T: EthSpec> {
     /// advanced forward one slot using `per_slot_processing`. This state is "primed and ready" for
     /// the application of another block.
     pub pre_state: BeaconState<T>,
+    /// This value is only set to `Some` if the `pre_state` was *not* advanced forward.
+    pub beacon_state_root: Option<Hash256>,
     pub beacon_block: SignedBeaconBlock<T>,
     pub beacon_block_root: Hash256,
 }
 
 impl<T: EthSpec> From<BeaconSnapshot<T>> for PreProcessingSnapshot<T> {
     fn from(snapshot: BeaconSnapshot<T>) -> Self {
+        let beacon_state_root = Some(snapshot.beacon_state_root());
         Self {
             pre_state: snapshot.beacon_state,
+            beacon_state_root,
             beacon_block: snapshot.beacon_block,
             beacon_block_root: snapshot.beacon_block_root,
         }
@@ -47,10 +51,15 @@ impl<T: EthSpec> CacheItem<T> {
     }
 
     pub fn into_pre_state(self) -> PreProcessingSnapshot<T> {
+        // Do not include the beacon state root if the state has been advanced.
+        let beacon_state_root =
+            Some(self.beacon_block.state_root()).filter(|_| self.pre_state.is_none());
+
         PreProcessingSnapshot {
             beacon_block: self.beacon_block,
             beacon_block_root: self.beacon_block_root,
             pre_state: self.pre_state.unwrap_or(self.beacon_state),
+            beacon_state_root,
         }
     }
 }