services/horizon: Add last_modified_time to account responses

[2opremio]

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 last_modified_time to account responses (making it consistent with offers).

last_modified_time is the closing time of the most recent ledger in which the account was modified.

Also, get rid of the account query consistency check, comparing Horizon's DB and Core's DB.

Why

Fixes #1945

Known limitations

It adds an additional DB query per account which can negatively affect performance.

[2opremio]

@bartekn PTAL

[tamirms]

looks good! I think we should update the changelog with a note about the new last_modified_time field in the account response

[2opremio]

I think we should update the changelog with a note about the new last_modified_time field in the account response

@tamirms I added it, but I wasn't sure what format to follow for unreleased changes in master. PTAL