Fixing race condition in HTTP/2 test

Motivation:

InboundHttp2ToHttpAdapterTest swaps non-volatile CountDownLatches in
handlers, which seems to cause a race condition that can lead to missing
messages.

Modifications:

Make CountDownLatch variables in InboundHttp2ToHttpAdapterTest volatile.

Result:

InboundHttp2ToHttpAdapterTest should be more stable.