[Altair] Sync committee pools (#2321)

Add pools supporting sync committees:
- naive sync aggregation pool
- observed sync contributions pool
- observed sync contributors pool
- observed sync aggregators pool

Add SSZ types and tests related to sync committee signatures.

Co-authored-by: Michael Sproul <michael@sigmaprime.io>
Co-authored-by: realbigsean <seananderson33@gmail.com>

beacon_node/store/src/partial_beacon_state.rs

@@ -6,6 +6,7 @@ use crate::{get_key_for_col, DBColumn, Error, KeyValueStore, KeyValueStoreOp};
 use ssz::{Decode, DecodeError, Encode};
 use ssz_derive::{Decode, Encode};
 use std::convert::TryInto;
+use std::sync::Arc;
 use types::superstruct;
 use types::*;
 
@@ -85,9 +86,9 @@ where
 
     // Light-client sync committees
     #[superstruct(only(Altair))]
-    pub current_sync_committee: SyncCommittee<T>,
+    pub current_sync_committee: Arc<SyncCommittee<T>>,
     #[superstruct(only(Altair))]
-    pub next_sync_committee: SyncCommittee<T>,
+    pub next_sync_committee: Arc<SyncCommittee<T>>,
 }
 
 /// Implement the conversion function from BeaconState -> PartialBeaconState.