Ensure consistent insertion for ConcurrentHashMap

Ensure instances of comparable and non-comparable classes can be inserted and found.

see discussion on concurrency-interest mailing list:
  http://cs.oswego.edu/pipermail/concurrency-interest/2013-July/011599.html
  http://cs.oswego.edu/pipermail/concurrency-interest/2013-June/011393.html

Can be verified with TCK test from:
  http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/test/tck/ConcurrentHashMapTest.java?r1=1.33&r2=1.34