[8u] Request for Approval for backport of 8166507: ConcurrentSkipListSet.clear() can leave the Set in an invalid state

[Martin Buchholz]

http://cr.openjdk.java.net/~martin/webrevs/openjdk8/ConcurrentSkipListMap-clear/

https://bugs.openjdk.java.net/browse/JDK-8166507

Patch from 9 applies cleanly.