Do not hold replication mutex when calling getMinValid

It is not valid to read collections while holding the replication mutex. The reason is that reading a collection takes the global lock, which may be held by another thread, and that thread may be waiting on the replication mutex (e.g. by calling getMyLastAppliedOpTime).

Thus, _replicationProcess->getConsistencyMarkers->getMinValid must not be called with the replication mutex held, as it is in _finishLoadLocalConfig.

I believe we may simply release the repl mutex before checking !lastOpTime.isNull()

https://github.com/mongodb/mongo/blob/960fca37a78040d75d87f8049ef4c4a6227506cc/src/mongo/db/repl/replication_coordinator_impl.cpp#L598

Then re-acquire after calling getMinValid() (and remove the else clause).