Bug 893838 - [Messages] Deleting threads and messages takes forever

https://bugzilla.mozilla.org/show_bug.cgi?id=893838

- Updates desktop mock:
    - Create an expensive thread of MMS messages
    - Mock delete support for array of message ids
    - Adds Z for Zombie, a console API for generating threads and messages
        - Z.threads(num, interval)
        - Z.messages(num, interval)
- Creates message caching
- Optimized, one step removal for all messages
    - Safe incoming messages in selectAll mode
- Optimized, one step removal for all threads
- Updates UI as priority, deletes actual messages asynchronously

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>