Remove SQL support for Ledger State

[SirTyson]

Description

Resolves #4501, #4502, and #4503

Rebased on #4403. This PR

1. Removes the flag for background eviction scan, making it mandatory
2. Removes the flag for SQL ledger state, making BucketListDB mandatory
3. Removes the --in-memory command line option.

Because of ongoing State Archival work that requires these changes, I've opted for a single branch to avoid rebasing complexity. That being said, I can open more granular PRs to assist in the reviewing process. In particular, 1 can be seperated into it's own PR.

Unfortunately, in order to make tests work without SQL ledger state tables, I've cannibalized --in-memory mode to be a testing only mode. This reuses most of the --in-memory code, but modifies it such that a node would not be able to sync with the network in this mode. For this reason, I think we need to deprecate SQL and --in-memory mode at the same time.

Finally, all tests pass except for "herder externalizes values". While I changed the TestDB mode for that test (as the old SQL mode is no longer supported), no other changes should have been made. I haven't been able to figure out why it's failing, but perhaps someone more familiar with Herder like @marta-lokhova could take a look.

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]

what does the herder failure look like? I checked CI logs, but it failed due to clang-format before it could get to the test.

[SirTyson]

what does the herder failure look like? I checked CI logs, but it failed due to clang-format before it could get to the test.

I pushed the formatting update, now the CI fails with the error I've been getting.