Fixes COMETD-458 (Seti: Duplicate messages, isPresent inconsistencies
 after multiple node crashes/reassociations).

Fixed a race condition where a presence message could have been
processed just after a "comet left" event, thus leaving the Seti with
 an association belonging to a Seti that does not exist anymore.
Now Seti uses a mechanism similar to OortObject to push its state to
a peer, and disassociates users if the node they have been associated
 is not connected anymore.