Ignore FETCH responses without UID

Exchange seems to send untagged FETCH responses without UID in response
to UID STORE requests.  (For the record, I think this is contrary to RFC
3501.)  Ensure we don't do bogus things when there is no UID.

This should probably trigger a flag update request or something, unless
I decide to map the sequence number to a UID.
Download flagged messages regardless of age