rhbz995964 - Delete Trans Memories asynchronously.

This is to prevent the container from cancelling long transactions while deleting large TMs. Also, the database might lock the table for too long.

This revision forces the user to clear the TM (remove all trans units) before letting them delete the TM. It is a bit of a nuisance, but given the time constraints it was the simpler way to go. We may revisit this later if it is too much of a pain to bush two buttons instead of one.