Minor post-merge cleanups

[zah]

https://github.com/status-im/nimbus-eth2/pull/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.

https://github.com/status-im/nimbus-eth2/pull/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.

[github-actions[bot]]

Unit Test Results

0 files   -        12  0 suites   - 860   0s :stopwatch: - 1h 6m 17s 0 tests  -   1 911  0 :heavy_check_mark:  -   1 764  0 :zzz:  - 147  0 :x: ±0  0 runs   - 10 341  0 :heavy_check_mark:  - 10 151  0 :zzz:  - 190  0 :x: ±0 

Results for commit 6ba7fba2. ± Comparison against base commit ede83b18.