The socket is now managed by the DomainSocketWatcher.  However,
we failed to pass it to the client.  We call shutdown() on the
UNIX domain socket now.  This will trigger the DomainSocketWatcher
callback.  The callback will close the domain socket.
We don't want to close the socket here, since that might lead to
bad behavior inside the poll() call.  See HADOOP-11802 for details.