Use _termShadow in updateTerm and processReplSetMetadata

In the Replication Coordinator, we currently take the replication coordinator lock whenever we update the term (every find and getMore) or receive a command with replSetMetadata (e.g. replSetUpdatePosition). In both cases we need only take the lock when the term we receive is greater than our current term, which we can determine by checking the existing atomic _termShadow.