“If the server does not know how to decode the section's CTE, it MUST
fail the request and issue a "NO" response that contains the
"UNKNOWN-CTE" extended response code.” — [RFC3516 section 4.3]

Unfortunately the client doesn't know which message couldn't be decoded,
so it can't fallback and use BODY instead.

This made ‘use-binary=NO’ pretty much mandatory.  Hence we remove
support for BINARY [RFC3516].  Instead, we increase the thresold for
when to add Zlib full flush points from 4096 to the buffer size (32768).

Also, add traffic statistics after closing the connection to the IMAP
server.

To avoid confusion with http://imapsync.lamiral.info .