refactor(history-network): rename master accumulator

[ogenev]

We will have more than one header accumulator for verifying the canonicalness of history headers:

• the well-known "master accumulator" for verifying pre-merge headers.
• the "Capella accumulator" (the frozen historical_roots from beacon state) for pre-Capella and post-merge headers.
• the "historical summaries" accumulator for post-Capella headers.

This PR proposes to rename MasterAccumulator to MergeAccumulator to make the name less confusing and more clear.

[ogenev]

I don't find MergeAccumulator particularly more clear.

It does not clarify that it is the accumulator for pre-merge data. And merge could also just mean something specifically regarding the accumulator itself.

What about PreMergeAccumulator, for pre-Capella headers it will be PreCapellaAccumulator (beacon state historical_roots) ?

Do you have any suggestions?

[kdeme]

What about PreMergeAccumulator, for pre-Capella headers it will be PreCapellaAccumulator (beacon state historical_roots) ?

Yes, PreMergeAccumulator would be better.

I would not create an additional name for the historical_roots and the historical_summaries. I'd just keep those names from the consensus specs.

[ogenev]

Yes, PreMergeAccumulator would be better.

Updated to PreMergeAccumulator.

[ogenev]

Added a new commit to rename theAccumulatorProof.