Before SERVER-58991, the 'step 7' of the MigrationDestinationManager timing object used to account up until the session cloner had finished. After SERVER-58991, in addition to that, it now also accounts for the time the MigrationDestinationManager waits for the release of its critical section.

We should restore the former semantics of 'step 7'; and introduce a new 'step 8', which exists only when the new migration protocol is in use that is stepped after the critical section has been exited.