RFR: 8048209 : (s) Collections.synchronizedNavigableSet().tailSet(Object, boolean) synchronizes on wrong object

Hello all; This change fixes an issue with the instance returned by Collections.synchronizedNavigableSet().tailSet(Object,boolean) which synchronizes on wrong object, itself rather than the same mutex as it's source. jbsbug: https://bugs.openjdk.java.net/browse/JDK-8048209 webrev: http://cr.openjdk.java.net/~mduigou/JDK-8048209/0/webrev/ Since I was concerned that there might be other lurking errors I opted to create a fairly thorough new test which, thankfully, revealed no additional problems. Mike