Geo: Successful sync should cause verification

This is needed to support verification of mutable types, like snippet repos: &2810 (closed)

This is also needed to support automatic healing of verification failures: #301244 (closed)

• On state transition to success, transition verification_state to verification_pending.
  • If transitioning from verification_failed then don't clear verification_retry_count, to ensure progressive backoff of syncs-due-to-verification-failures

I extracted this out of #301244 (closed) so that issue could be moved out of &1817 (closed) if desired.