Bug 823521 - [Email] Deleting messages from the trash-folder deadlocks the back-end, breaks mail client in that folder

https://bugzilla.mozilla.org/show_bug.cgi?id=823521

This has r+ from @asutherland over at https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/115 (see that PR for unit tests, as well).