[8u] RFR 8202788: Explicitly reclaim cached thread-local direct buffers at thread exit

[Zhengyu Gu]

I would like to backport this patch to openjdk8u for parity with Oracle 
8u321.

Bug: https://bugs.openjdk.java.net/browse/JDK-8202788
Patch: http://hg.openjdk.java.net/jdk/jdk/rev/106dc156ce6b

The patch does not apply cleanly.

- Fixed instantiating generic classes due to inferring type arguments
- NativeBuffer does not have free() method, replaced with 
buffer.cleaner().clean()
- Conflicts on imports in NativeBuffers.java and Util.java

8u webrev: http://cr.openjdk.java.net/~zgu/JDK-8202788-8u/webrev.00

Test:
   Passed all three tests in patch.

Thanks,

-Zhengyu