Mitigate access to messages on slow IMAP servers.

Mitigate access to messages on slow IMAP servers.

* org-gnus.el (org-gnus-nnimap-query-article-no-from-file): New
customization variable.
(org-gnus-nnimap-cached-article-number): New function.
(org-gnus-follow-link): Try to fetch cached article number of
message-id.

Some IMAP servers (e.g. Courier) are slow when searching for a message
by its message id header field.  Because article numbers in IMAP
mailboxes are persistent UIDs, we can try to look up the UID of a IMAP
message in Gnus' cache for the mailbox in question and skip the slow
search on the server.

The problem with slow server was reported by Sébastien Vauban and the
patch is based on the work of Tassilo Horn.