Put validator pubkey cache back in beacon_chain

[michaelsproul]

Description

At one point in the history of tree-states I relocated the validator pubkey cache into the store crate. With the latest database schema that's no longer necessary, so I think it would make sense to minimise the size of the (source code) diff and move it back. We just need to take care to preserve the changes which implement #3505.

[michaelsproul]

I'd forgotten that the pubkey cache is used in the database for the storage of full states (store_full_state/get_full_state). For simplicity and to future-proof against validator index re-use in future versions of the protocol I think we should give up on trying to de-duplicate the validator pubkeys from the states. We only ever store a handful of full states at a time, ~N in the freezer database for the last N years of chain history, and 1 in the hot database for the finalized state.

Therefore I think we can remove ValidatorTrait and CompactBeaconState, and move the validator pubkey cache back to beacon_chain. This will be a breaking schema change for tree-states because the state format on disk will change.