services/horizon: Implement horizon db fill-gaps command

[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

Close #4010

horizon db fill-gaps can be called with no parameters in which case it queries for any existing gaps in the horizon db and then proceeds to ingest history to fill the gaps.

The command can also be called with a start and end ledger parameter. In which case, the command will only query for gaps within the provided range.

horizon db fill-gaps 1 1000 will fill any gaps occurring within the range 1-1000

It is also possible to configure the same parallel worker command line parameters that are used by horizon db reingest range so that fill-gaps can be also take advantage of concurrency.

Known limitations

[N/A]

[tamirms]

@bartekn can you take another look?