HTTP/2 Unit Test Leak Fixes

Motivation:
The HTTP/2 tests do not always clean up ByteBuf resources reliably. There are issues with the refCnt, over allocating buffers, and potentially not waiting long enough to reclaim resources for stress tests.

Modifications:
Scrub the HTTP/2 unit tests for ByteBuf leaks.

Result:
Less leaks (hopefully none) in the HTTP/2 unit tests.  No OOME from HTTP/2 unit tests.
I'm surprised there isn't something like this already in Netty.

@normanmaurer should we add this to ReferenceCountUtil?