Several changes that make IMAIL come up much faster with large

Several changes that make IMAIL come up much faster with large
folders.  First, we pay attention to the UNSEEN value, and delete
UNSEEN when expunges happen to avoid having an invalid value.  Second,
we don't load headers for all the messages in advance; just their
UIDs, which is very fast.

This allows the mail reader to fetch just the information for the
first unseen message, which happens very quickly on a fast network
connection.