Allow healthy databases to skip repairs

As a complement to SERVER-29553, allow me to skip repairs on an already repaired database. The aforementioned database with hundreds of millions of documents took a couple of hours to repair, and it's now healthy.

Unfortunately, while attempting to repair some another database in this instance I hit some assertion failure which stopped the entire repair process, rather unnecessarily, i think. SERVER-29555

Now when I restart the repair, I'm going to waste another few hours waiting for a_big_database to repair unnecessarily before I can get around to repairing some_other_big_database which is what I really care about.