Update `eth1` monitor for beacon node.

[ralexstokes]

The current eth1 follower needs to be brought up to spec but this work is outside the current scope of getting a "read-only" node on the eth2 testnets.

This issue will track stuff that should be considered when updating the eth1 follower:

[ralexstokes]

Consider:

Include validity check for this condition: ethereum/consensus-specs#1834

[ralexstokes]

Double check this: ethereum/consensus-specs#1756