MINOR: quic-stream: use distinct tree nodes for quic stream and qcs

MINOR: quic-stream: use distinct tree nodes for quic stream and qcs

Simplify the model qcs/qc_stream_desc. Each types has now its own tree
node, stored respectively in qcc and quic-conn trees. It is still
necessary to mark the stream as detached by the MUX once all data is
transfered to the lower layer.

This might improve slightly the performance on ACK management as now
only the lookup in quic-conn is necessary. On the other hand, memory
size of qcs structure is increased.