Fixes Issue 1551

Fixes Issue 1577

Issue 1551: 
Some IMAP servers send untagged EXPUNGEs to IDLEing
clients without ever haven't sent an untagged FETCH.  The untagged
EXPUNGEs are harder to deal with because they don't have a UID.  So,
if the user has elected to have the IDLE connection start with a poll,
we can maintain a map of message sequence numbers to UIDs that we can
use to figure out which message to delete.  To mitigate the risk of
the map falling out of date, we do a UID SEARCH UID before removing
the local copy of the message, just to make sure the message is really
gone from the server.  If we detect an error, do another poll to
resync the map.

Issue 1577:
Restore the removal of notifications for an account when the account's
unread message count goes to 0.