This will restore graceful shutdown of workers by master.
Also worker_int callback is moved from handle_exit (INT and TERM before switching) to handle_quit(INT and QUIT).
Commit 81241907ffcf94517ffa14b8427205906b61b540 changed the signal
handling by switching the roles of `TERM` and `QUIT` for the arbiter
so that `TERM` is graceful and `QUIT` is not.
At the time, workers performed graceful shutdown on `QUIT` and quick
shutdown on `TERM` and `INT`. This behavior was also changed so that
`QUIT` (and `INT`) cause a quick shutdown and `TERM` is graceful.
However, the documentation incorrectly reversed the roles of the worker
signals and the arbiter was not updated to use the correct signals.
This commit fixes the documentation and the arbiter signals.
