Close existing sockets in master process when stopping.

Previously, when `gunicorn` is told to restart, it'll signal
workers to stop listening for new connections. But listening
ports are still open in master process. This lets load balancer,
such as haproxy, think that this server is still up and dispatch
requests to this server. But because all workers have stopped
accepting new connections. New requests will block until timeout.
This is especially severe when conducting a restart in a setup with
long graceful-timeout setting, causing many requests to timeout.

This patch fix that by closing all `LISTENERS` in `stop` method of
`Arbitor`.
The problem is during the process of shutdown. As i said in the pull request description, if listening ports are not closed, haproxy will still regard this server as a healthy server, until vm have all stopped. Hence there's no log from haproxy, because it thinks everything is still normal.

> Anyway we should make sure that closing here the socket will not make the worker crashing in an attempt to listen on the socket. 

But master is already requiring workers to `stop`.... Or maybe I should close listening sockets after sending signal?