cluster: a normal disconnect should not error

Its not an error for the worker to disconnect itself, but v0.11 sends
more messages over the ipc channel, so disconnect was causing errors to
be thrown in situations that v0.10 would not error. Its possible that
messages related to cluster will never get responses now, but since a
disconnected worker is about to exit this should not matter.
cluster: fix api breakage related to suicide

Fix inadvertent v0.11 changes to the definition of suicide, particularly
the relationship between suicide state, the disconnect event, and when
exit should occur.

Workers shouldn't normally exit on disconnect, it doesn't give them time
to do a graceful finish of open client connections. But on unexpected
disconnect they should exit so the workes aren't left around after the
master.
!fixme _test-cluster.sh
!fixup emit 'setup' in nextTick
cluster: modify test to pass on v0.10

This test as-written was invalid, it fails against the v0.10 cluster
API, and the commit that introduced it didn't document any intentional
change to the cluster API.