JGRP-1837 Stopping FD_SOCK takes too long

https://issues.jboss.org/browse/JGRP-1837

Clear the pingable_mbrs collection to signal the pinger thread that
it should stop, in addition to closing the socket and interrupting
it.

The difference is quite visible on the Infinispan test suite, it got it down from 8:22 mins to 7:18 mins for me.

I tried to do something similar in GMS (ViewHandler.run()), as it also seemed to take longer than expected. But I think that might be just because of the coordinator trying to broadcast a view without itself to the other members before leaving.