(nnimap-verify-uidvalidity): Fixed bug where uidvalidity
wasn't updated after mismatch. Clear cached mailbox info correctly
when uidvalidity changes.
(nnimap-group-prefixed-name): New function to avoid some code duplication.
(nnimap-verify-uidvalidity, nnimap-group-overview-filename)
(nnimap-request-group): Use it.
(nnimap-retrieve-groups, nnimap-verify-uidvalidity)
(nnimap-update-unseen): Significantly improved speed of Gnus startup
with many imap folders. This is done by caching the group status from
the imap server persistently in a group parameter `imap-status'. (This
was cached before too if `nnimap-retrieve-groups-asynchronous' was set,
but not persistently, so every Gnus startup was still very slow.)