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.
On the below lines (sorry won't let me create a comment on them) are the following OK to do

```java
unallocatedBytes -= nextState.unallocatedPriorityBytes();
remainingWeight -= weight;
```

If the `if` statement on line 357 is false?