Major IMAP IDLE rework:

1) Actual message sync on MessagingController uses same connection as
   IDLE for faster and more efficient push operation.  Uses fewer
   connections to the server.

2) More aggressive handling of untagged responses should more reliably
   get flag changes and new messages when many events happen at once.

3) Simplification of new mail notification

4) Push mail now respects the folder visible limit

5) When multiple untagged FETCH responses arrive en bloc, the actual
   message flags and UIDs are fetched with a single request