There are two checks that need to be performed:
1. the crypto stream must not have any more data queued for reading
2. when receiving CRYPTO frames for that crypto stream afterwards, they
must not exceed the highest offset received on that stream
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.
