cluster: only forcibly exit worker on unclean exit

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

In v0.10, workers don't forcibly exit on disconnect, it doesn't give
them time to do a graceful finish of open client connections, they exit
under normal node rules - when there is nothing left to do. But on
unexpected disconnect they do exit so the workers aren't left around
after the master.

Note that a test as-written was invalid, it failed against the v0.10
cluster API, demonstrating that it was an undocumented API change.