Fixes Issue 827

Fixes Issue 734

On reconnection, IMAP IDLE folders will now fetch up to the most
recent 10 message that arrived while a IDLE connection was not
available.

Fixes Issue 232

A serving of NAMESPACE-based auto-configuration on the side.  If the
IMAP prefix is empty, and the IMAP server supports NAMESPACE, use the
IMAP prefix supplied by NAMESPACE.  Also, if the user manually puts
the separator as the last character in the prefix, don't append the
separator.

Also:
Improved reliability of IMAP IDLE incoming message and flag state
change handling.  Reduction (but not elimination) of multiple
connections under startup conditions.