Improved contact presence support:

1. Support for long lists of user names by sending multiple queries.
2. Created FIFO list for nick sets. Each nick set is exactly the content
of 1 query, such that we can handle the RPL_ISON events by comparing
expected nick list with actual nick list.
3. Update presence upon receiving QUIT, channel join.
4. Escape for case when unexpected RPL_ISON is received. (i.e. list of
expected nick sets is empty)
5. Implemented subscribe, unsubscribe methods for
OperationSetPersistentPresence.
Tweaks.
Make submittedMessage volatile, just in case.
Converted sorted set to list, since we have a (already sorted) list
of nicks that we query for in a query message. (+ misc improvements)
Clean up.
Watch list now persistent over (dis)connects.

The watch list is now persistent over disconnects, such that
disconnecting and reconnecting to the IRC account will not lose the nick
watch list content. Of course, we will still lose all contacts when
Jitsi is shut down, because the contacts are volatile contacts.