Review flow control for Transfer-Encoding: chunked

[jfallows]

We currently extend the WINDOW budget each time a new chunk is discovered.

Then at the end of the response, when the stream is preparing for the next response, the WINDOW budget is extended to the maximum headers size which is a full buffer slot.

We can instead decode the chunked response in blocks, up to one full buffer slot at a time, taking care not to exceed the available WINDOW budget of the receiving application HTTP response stream.

This will result in a smaller number of larger size WINDOW credits being sent to the network HTTP response stream.

[jfallows]

@cmebarrow @jitsni I believe we've already taken care of this - please confirm.