Don't crash when updating excluded message

If the ewoc node has been removed (probably by an "only unread"
restriction), don't crash.  Also make the hash table weak, so that stale
entries expire.
Download flags for existing messages

Do it the naïve way first: ask for flags for every message we know
about.  We should use modseq if the server/mailbox supports it.
Add capability check function; ask for server ID

The ID will only turn up in the transcript.  Maybe this will be useful
for debugging.