By introducing a callback to the stream, which the stream calls as soon
as it is completed, we can get rid of checking every single open stream
if it is completed.
The queue holds all streams that have data to send. When stream.Write or
stream.Close are called, a stream is added to this queue. It is removed
from the queue when all available (at that moment) data was sent.
This way, we don't need the round robin scheduling (which, for every
packet sent, asked every single open stream if it had data) any more.
