Remove SQL for (most) Ledger State

[SirTyson]

Description

Resolves #4503 and #4502

This change removes SQL support for all ledger state, except for the offers table. It also enforces background eviction. In order to not break Horizon, core will not crash if any of the old flags or config settings are used. Instead it will print a warning and ignore the flags. The flags should be removed in the next protocol release.

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

[marta-lokhova]

very nice to see so much legacy code go away, thank you! I left a few questions wrt the migration, and a couple of minor nits. Otherwise happy to merge this asap.