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.
Does it make sense to change this to conditionally flush now that we have the byte count?

```java
if(writeAllowedBytes(connectionStream, state(connectionStream).priorityBytes()) > 0) { flush()); }
```