IMAP command batching code, used to pipeline mailbox poll requests.

IMAP command batching code, used to pipeline mailbox poll requests.
Up to 10 poll commands will be sent at a time (tunable in imap_private.h).
This is a huge win on my currently awful wireless link. It takes a knife
to a lot of fundamental IMAP code (mostly for the better), so it may
have destabilised things. Time for some brave (or lazy non-Changelog-reading)
testers to report... next up, IDLE support.