Fix bug in HTTP/2 outbound flow control

Motivation:

The outbound flow controller logic does not properly reset the allocated
bytes between successive invocations of the priority algorithm.

Modifications:

Updated the priority algorithm to reset the allocated bytes for each
stream.

Result:

Each call to the priority algorithm now starts with zero allocated bytes
for each stream.
Understood that the situation depends upon how it is used but typically an effort to throw a `ConcurrentModificationException` is made in these situations.  I'm fine with assuming that the behavior undefined and not throwing anything but just wanted to get your feeling if we need to emulate the JRE iterator behavior.  I think if we were to do anything about it we would put the detection in [IteratorImpl](https://github.com/nmittler/netty/blob/http2_outbound_flow_bug/common/src/main/java/io/netty/util/collection/IntObjectHashMap.java#L443).