Add AttesterCache for attestation production (#2478)

## Issue Addressed

- Resolves #2169

## Proposed Changes

Adds the `AttesterCache` to allow validators to produce attestations for older slots. Presently, some arbitrary restrictions can force validators to receive an error when attesting to a slot earlier than the present one. This can cause attestation misses when there is excessive load on the validator client or time sync issues between the VC and BN.

## Additional Info

NA

beacon_node/beacon_chain/src/lib.rs

@@ -1,5 +1,6 @@
 #![recursion_limit = "128"] // For lazy-static
 pub mod attestation_verification;
+mod attester_cache;
 mod beacon_chain;
 mod beacon_fork_choice_store;
 mod beacon_proposer_cache;