Mark MigrationDestinationManager::_migrateThread killable for stepdown

MigrationDestinationManager::_migrateThread() is not marked killable. We should probably do something like what we did for the range deleter to call setSystemOperationKillable() on the client thread. Otherwise, the migrateThread could be blocked on a prepared transaction for a collection lock (held by a prepared transaction) while holding onto the RSTL, preventing state transition from proceeding. Calling setSystemOperationKillable() will allow stepDown to kill the migrateThread operation.