Minor post-merge cleanups

#3944

The use of nested `awaitWithRetries` calls would have
resulted in an unexpected number of retries (3x3).
We now use regular `await` in outer layer to avoid the problem.

#3943

The new code has an invariant that the `headMerkleizer` field in
the `Eth1Chain` is always kept in sync with the blocks stored in
the chain.

This invariant is now enforced better by doing the necessary merkleizer
updates in the `Eth1Chain.addBlock` function.