OLDNAME is included in IMAP4rev2, but not in IMAP4rev1. it is also included in
the NOTIFY extension, but we don't implement that yet.
found by Damian Poddebniak with https://github.com/duesee/imap-flow, thanks!
renaming inbox is special. the mailbox isn't renamed, but its messages moved to
a new mailbox. we weren't updating the destination mailbox uidnext with the new
messages. the fix not only sets the uidnext correctly, but also renumbers the
uids, starting at 1.
this also adds a consistency check for message uids and mailbox uidnexts, and
for mailbox uidvalidity account nextuidvalidity in "mox verifydata".
this also adds command "mox fixuidmeta" (not listed) that fixes up mailbox uidnext
and account uidvalidity. and command "mox reassignuids" that will renumber the
uids for either one or all mailboxes in an account.
