RFR(S): 8204857: ConcurrentHashTable: Fix parallel processing

[Robbin Ehn]

Hi, please review,

The parallel processing functionality of the concurrenthastable is not working.
This adds a gtest for it and fixes the issue. But there are no users of it, so 
the most important is the gtest so we don't breaks this in the future.

Change-set: http://cr.openjdk.java.net/~rehn/8204857/
Bug: https://bugs.openjdk.java.net/browse/JDK-8204857

/Robbin