Draft: Recover SCP messages when replaying from history

[bboston7]

Description

Resolves #3512

This PR enables updating the scphistory table during catchup from history. It allows users to specify which archives to use via the SCP_HISTORY_ARCHIVES config option. If a user specifies multiple archives, stellar-core will merge the messages from the archives.

This is a draft PR because I'm looking for feedback on the approach, but still have some work to do before it is in a mergeable state. Most of the remaining work is to:

• Document the new functionality
• Write additional tests for:
  • Merging
  • Failed downloads
  • No SCP_HISTORY_ARCHIVES
  • Multiple SCP_HISTORY_ARCHIVES
  • Changes to ReplayDebugMetaWork
• Integrate changes from #4121
• Address other TODOs in the changes

Checklist

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