Change the operator order of TrafficCounter to calculate the throughput to get the correct result

Motivation:
Currently the last read/write throughput is calculated by first division,this will be 0 if the last read/write bytes < interval,change the order will get the correct result

Modifications:
Change the operator order from first do division to multiplication

Result:
Get the correct result instead of 0 when bytes are smaller than interval
Motivation:

The HTTP/2 tests have been unstable, in particular the
Http2ConnectionRoundtripTest.

Modifications:

Modified fields in Http2TestUtil to be volatile.

Result:

Tests should (hopefully) be more stable.
HTTP/2 unit test missed syncrhonized collection

Motiviation:
PR https://github.com/netty/netty/pull/2948 missed a collection to synchronize in the HTTP/2 unit tests.

Modifications:
synchronize the collection that was missed

Result:
Missed collection is syncronized and initial size is corrected
Adding missing asserts to HTTP/2 tests

Motivation:

Some tests do not properly assert that all requests have been
sent/received, so the failures messages may be misleading.

Modifications:

Adding missing asserts to HTTP/2 tests for awaiting requests and
responses.

Result:

HTTP/2 tests properly assert message counts.