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.
Merge pull request #122 from zanata/async-tm-delete

rhbz995964 - Delete Trans Memories asynchronously.
Merge commit '54414f45fe323833b8490124b5cea2ab271d06ba' into HEAD