Fix race on startup with HTTP server

[SirTyson]

Description

Resolves #5099

This PR adds a check to HTTP endpoints and exits early if core is not in the correct state to service the endpoint. I've audited the endpoints and divided them into three categories:

• Ledger State Dependent Endpoints (i.e. anything with a LedgerHeader or BucketList dependency). Here, we check that LedgerManager is not in LM_BOOTING_STATE such that there is some valid LCL state to query. These endpoints include:

  • /tx
  • /sorobaninfo
  • /upgrades
  • /dumpproposedsettings
  • /self-check
  • /maintenance
  • /getledgerentryraw
  • /getledgerentry
  • /info

• Herder Dependent Endpoints. These don't need ledger state, but require overlay and herder to be initialized:

  • /peers
  • /connect
  • /droppeer
  • /bans
  • /unban
  • /quorum
  • /scp
  • /stopsurvey
  • /getsurveyresult
  • /startsurveycollecting
  • /stopsurveycollecting
  • /surveytopologytimesliced

• No dependencies. These include all test-only endpoints for simplicity, as well as diagnostic info that should probably always be live, such as /metrics and /ll.

I thought about just delaying starting the endpoint (like the comment initially mentioned), but given the current length of our startup time, we need endpoints like /metrics available at a minimum. It is also probably better from a consumer view for captive-core to reply with a "not ready" status instead of less graceful errors.

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