Process attestations when the state from their target block state is available in cache

[ajsutton]

PR Description

When selecting a state to use to process attestation gossip, check if the attestations beaconBlockRoot state is already available in the cache before deciding if it's worth regenerating a state. This allows us to process significantly more attestations without requiring additional state generations - particularly if attestations are received for blocks we just imported on a long fork.

Also potentially reduces the number of generations we'd do during periods of non-finality by better leveraging what we have in cache.

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[rolfyone]

LGTM! Nice to see you :)