Hot Archive merge into Cold Archive

[SirTyson]

Description

Partially resolves #4396

When the HotArchive BucketList becomes full, it needs to be merged into a single ColdArchive Bucket. This bucket will be used to initialize the ColdArchive BucketList that produces the root merkle hash for the given archival trie.

This PR adds functionality to merge a completed HotArchive into a single ColdArchive Bucket. For simplicty, this is not done using the FutureBucket interface. Due to this, we can not pick up in-progress merges across node restarts. If a merge is not completed and the node restarts, the node will have to restart the merge. However, if a node finishes the merge then restarts, it will pick up the finished bucket on restart. I think for now this is fine. The HotArchive BucketList will be small such that merges are quick. If this is an issue, it can always be addressed later.

Opening as a draft until #4403 is merged.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document