- JGRP-1837 Stopping FD_SOCK takes too long
  Clear the pingable_mbrs collection to signal the pinger thread that
  it should stop, in addition to closing the socket and interrupting
  it.
- Stopping DiagnosticsHandler correctly for fast disconnect() (https://issues.jboss.org/browse/JGRP-1837)
- Stopping socket handler in UDP quickly