- Use trrigers to keep them up-to-date.
- Batch-update them upon upgrade.
- Motivation:
On the mailbox list, we show the number of messages in trash/drafts.
We currently do this with count(*) on the fly, which is okay
because MailboxList is really shown right now.
However, on the 2 pane, it's always shown and constantly refreshed,
so the use of count(*) can be a huge penalty.
It also make the code significantly simpler.
Change-Id: I26efa238d5183df43420a65925876248ef6c6cb6
574854b528