Make ESEARCH UID results explicitly omit non-existent UIDs from results.
Make ESEARCH UID results explicitly omit non-existent UIDs from results.

This removes an optimization that Mark Crispin fought to retain in his code, but it
matches the expectation that a number of IMAP clients in the wild depend on.

jonabbey/panda-imap,jonabbey/panda-imap,jonabbey/panda-imap