IMAP: Better handling of missing UIDNEXT response in RetrieveItemsTask

This improves experience with Courier IMAP servers which does not return UIDNEXT in
SELECT response. That forced us to always refetch the entire mailbox. Courier however
does return UIDNEXT in response to STATUS, so we now issue a STATUS command in case
SELECT gives us empty UIDNEXT.

REVIEW: 127457
BUG: 338186
FIXED-IN: 16.04