Attestations triggered by head events

[realbigsean]

Issue Addressed

Resolves: #2503

Proposed Changes

• Track which fallback node we are likely to use in our next attestation production
• Create head event subscriptions to all nodes provided by --beacon-nodes, if any of these subscriptions end, try to re-establish them at the beginning of the next slot
• Before 1/3 through the slot:
  • If we get a head event from the node we are like to produce our next attestation with, begin the attestation production process
  • Compare the head root we received in the head event to the head root in the AttestationData response from the beacon node. If they are different, wait until 1/3 of the way through the slot and retry
• At 1/3 through the slot:
  • Existing flow

Additional Info

TODO:

• [x] Add the same logic for sync committees
• [ ] Tests
• [x] Add flag to enable/disable (not sure we want this forever, but think it'll be useful for testing)