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/upgrade/capella.rs

@@ -1,6 +1,7 @@
 use std::mem;
 use types::{
-    BeaconState, BeaconStateCapella, BeaconStateError as Error, ChainSpec, EthSpec, Fork, VList,
+    BeaconState, BeaconStateCapella, BeaconStateError as Error, ChainSpec, EpochCache, EthSpec,
+    Fork, VList,
 };
 
 /// Transform a `Merge` state into an `Capella` state.
@@ -66,6 +67,7 @@ pub fn upgrade_to_capella<E: EthSpec>(
         committee_caches: mem::take(&mut pre.committee_caches),
         pubkey_cache: mem::take(&mut pre.pubkey_cache),
         exit_cache: mem::take(&mut pre.exit_cache),
+        epoch_cache: EpochCache::default(),
     });
 
     *pre_state = post;