Monitor and wait for 'DOWN' from our own GM as well as the slaves, to make sure our GM does not try to update group info after we have called forget_group/1. Also ensure we unlink from the coordinator when shutting down as well as when stopping mirroring, not strictly necessary but clearer.
Actually let's move the unlink/1 back, not convinced it is any clearer.
Merge bug26102
ensure callback_view_changed is called whenever the view may have changed
We had missed a case here.

Ditto for check_neighbours.
less lazy death notifications
When we receive a DOWN, it's possible that there are no messages from
the dead member floating around anyway. So rather than waiting for
some other activity to occur first, it is perfectly safe enough to
always call maybe_erase_aliases. That way we get slightly less lazy
death notification in this case (i.e. we can then be certain that
either the dead member is fully removed immediately or there must
still be messages from the dead member floating around. No other
possibility exists).
ensure propagation of master death
Previously slaves were monitoring the master and sending a
process_death message through gm in an attempt to get the updated gm
state propagated (and thus members_changed callbacks getting invoked
on slaves, which in turn trigger promotion).

However, the DOWN notification for the master may well arrive and be
processed, and the process_death message sent around the ring, before
GM has noticed the master death, thus rendering the process_death
broadcast ineffectual for its intended purpose.

With the recent GM changes we can guarantee that at least one slave
will be notified of the master death via GM. So we emit the
process_death message then, and not on master DOWN (and ditch the
master monitoring as a result). Since the new emission is triggered by
a gm notification we know GM is aware of the master death, thus
avoiding the aforementioned problem.
eliminate comment duplication
Merge bug26084