[Merge] Only subscribe to block gossip during optimistic sync

[ajsutton]

Description

When optimistic sync is enabled, Teku should ensure it is not subscribed to any gossip except the block gossip topic. Other topics require the block to be fully validated so we are unable to forward gossip while optimistically syncing and would risk being downscored if we stayed subscribed.

On the initial startup, teku only enables gossip once the head block is close enough to the current slot to avoid this same problem because we haven't yet processed blocks. This check needs to be changed to use the optimistic head to enable gossip, but then only subscribe to the block channel until our head block is verified.