Bug 1053952 - [Messages][Refactoring] Delete all messages at once when deleting threads. r=schung

Earlier ```Drafts.store()``` was called twice (https://github.com/mozilla-b2g/gaia/commit/87f4976ec3213e56dd6ec4fc98a4cc4074508916), after draft deletion and after thread deletion, I'm not sure why did we need to call it in the second case, do you have any ideas?