Add caching for state.eth1_data_votes (#919)

## Issue Addressed

NA

## Proposed Changes

Adds additional tree hash caching for `state.eth1_data_votes`.

Presently, each time we tree hash the `BeaconState`, we recompute the `state.eth1_data_votes` tree in it's entirety. This is because we only previous had support for caching fixed-length lists.

This PR adds the `Eth1DataVotesTreeHashCache` which provides caching for the `state.eth1_data_votes` list. The cache is aware of `SLOTS_PER_ETH1_VOTING_PERIOD` and will reset itself whenever that boundary is crossed.

This cache adds a new (but somewhat fundamental) restriction to tree hash caching:

*For some state `s`, `s.tree_hash_cache` is only valid for `s` or descendants of `s` that have been reached via state transitions that are faithful to the specification (invalid blocks are permitted, as long as they are faithfully processed).*

testing/ef_tests/tests/tests.rs

@@ -180,8 +180,8 @@ mod ssz_static {
     ssz_static_test!(
         beacon_state,
         SszStaticTHCHandler, {
-            (BeaconState<MinimalEthSpec>, BeaconTreeHashCache, MinimalEthSpec),
-            (BeaconState<MainnetEthSpec>, BeaconTreeHashCache, MainnetEthSpec)
+            (BeaconState<MinimalEthSpec>, BeaconTreeHashCache<_>, MinimalEthSpec),
+            (BeaconState<MainnetEthSpec>, BeaconTreeHashCache<_>, MainnetEthSpec)
         }
     );
     ssz_static_test!(checkpoint, Checkpoint);