FILE_PING: options to remove zombies

In FILE_PING and subclasses (S3_PING, GOOGLE_PING), coordinators write the files (e.g. A.list for coord A).
There's a shutdown hook that removes A.list when A crashes.
However, when a coordinator is killed by kill -9, the file A.list won't get removed.
The problem with this is that new members will read A.list and get delayed trying to ask A to join the cluster although A's not alive anymore (B is and created B.list).
Possible solution: implement a mechanism similar to JGRP-1915 where a coordinator removes all files on a view change with leaving members, and then writes its file again.