services/horizon: deprecate in-memory mode

[mollykarcher]

What problem does your feature solve?

Core team wants to deprecate in-memory mode. It's burdensome to maintain and not well-tested on their side when new features roll out.

What would you like to see?

Deprecate the captive-core-use-db flag. Meaning, emit a deprecation warning when it is sent on the command line.

At a later date, we'll remove it. When this issue is closed, we should create the future issue to actually remove the flag.

What alternatives are there?

• Remove it, rather than going through a lengthy deprecation process. Deprecation has little value being that there is no action to take on the part of the user when they see that this flag is deprecated