swim_quit() notifies all the members that this instance has left
the cluster. Strangely, except self. It is not a real bug, but
showing 'left' status in self struct swim_member would be more
correct than 'alive', obviously.

It is possible, that self struct swim_member was referenced by a
user - this is how 'self' can be available after SWIM instance
deletion.

Part of #3234