services/horizon/internal: Add flag to enable / disable reaping of lookup tables

[tamirms]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Add flag to enable / disable reaping of lookup tables.

Why

Reaping of lookup tables is an operation which blocks the ingestion thread and it can be a lengthy operation. We need to have a mechanism to disable reaping of lookup tables while still enabling reaping of history so we can prevent ingestion lag from occurring when reaping of lookup tables takes too long.

Note that it is safe to run horizon with reaping of history enabled and reaping of lookup tables disabled. The only negative consequence of such a configuration is that there may be rows in the lookup tables which are not referenced anymore (e.g. a history account id which is no longer used in any of the history tables). But, once reaping of lookup tables is re-enabled those rows should eventually be deleted.

Known limitations

[N/A]