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_block_processing.rs

@@ -41,6 +41,7 @@ mod verify_proposer_slashing;
 use crate::common::decrease_balance;
 use crate::StateProcessingStrategy;
 
+use crate::epoch_cache::initialize_epoch_cache;
 #[cfg(feature = "arbitrary-fuzz")]
 use arbitrary::Arbitrary;
 
@@ -114,6 +115,9 @@ pub fn per_block_processing<T: EthSpec, Payload: AbstractExecPayload<T>>(
         .fork_name(spec)
         .map_err(BlockProcessingError::InconsistentStateFork)?;
 
+    // Build epoch cache if it hasn't already been built, or if it is no longer valid
+    initialize_epoch_cache(state, state.current_epoch(), spec)?;
+
     let verify_signatures = match block_signature_strategy {
         BlockSignatureStrategy::VerifyBulk => {
             // Verify all signatures in the block at once.