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.
Ok ;).  While you at it I noticed 2 usages of the existing children() method that are unnecessary.
Can you change the following 2 lines [DefaultHttp2Connection.java#L424](https://github.com/netty/netty/blob/master/codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2Connection.java#L424) and [DefaultHttp2Connection.java#L427](https://github.com/netty/netty/blob/master/codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2Connection.java#L427) to use `newParent.numChildren()` instead of `newParent.children().size()`?