Improve QoS

[Stebalien]

Currently, when sending a lot of data, a single stream can fill the queue with 64*64*1024 = 4MiB of data to send. Ideally, we'd have per-stream QoS and a stream that just wants to send a few bytes would be able to jump this queue.

[Stebalien]

Simple solution: Just add another window like the sendWindow but make it a much smaller "queue" window.