Add a UNIQUEID property to mailboxes

[brong]

As in #6, unique IDs on things are good. We should provide a STATUS and SELECT field for showing a UNIQUEID which is retained when a folder is renamed.

Key considerations:

• If UNIQUEID is changed them UIDVALIDITY MUST also change, but not vice-versa.
• Server SHOULD keep the same UIDVALIDITY when renaming a folder (particularly useful if the client offers UNDO in its UI and can reverse the rename by renaming back again - if the UIDVALIDITY persists through the two renames, it will look identical to old clients that didn't fetch during the gap)

Again, I would argue for an opaque string that has space to store a UUID - so 70 bytes or more as the maximum size.

[brong]

Have written draft-gondwana-imap-uniqueid