ingest/ledgerbackend: Check if there are newer ledger when requested ledger does not exist

[bartekn]

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

In DatabaseBackend.GetLedger(ctx, sequence) check if there are newer ledgers than sequence if ledger not found. If there are newer ledgers return an error.

Why

If Stellar-Core cursors were manually removed or updated, it's possible it run a maintenance removing older ledgers. In such situation DatabaseBackend.GetLedger would loop indefinitely waiting for a ledger to appear in the DB but it never will.

Known limitations

No tests because there is no test suite for DatabaseBackend.