ContextClassCache uses ConcurrentHashMap w/concurrent performance test

I was suspicious that locking was faster than ConcurrentHashMap, so I
wrote a test to exercise the system under high concurrency, and compared
this implementation with the implementation it replaced. The difference
on my machine was:

Previous version:
Average: 6.496200 ms +/- 5.524512 ms
Total elapsed = 40640 ms (avg = 0.812800 ms)

This version:
Average: 4.864160 ms +/- 5.600783 ms
Total elapsed = 30457 ms (avg = 0.609140 ms)

So quite a bit better performance under concurrent load.