MEDIUM: mux-quic: implement attach for new streams on backend side

MEDIUM: mux-quic: implement attach for new streams on backend side

Implement attach and avail_streams mux-ops callbacks, which are used on
backend side for connection reuse.

Attach operation is used to initiate new streams on the connection
outside of the first one. It simply relies on qcc_init_stream_local() to
instantiate a new QCS instance, which is immediately linked to its
stream data layer.

Outside of attach, it is also necessary to implement avail_streams so
that the stream layer will try to initiate connection reuse. This method
reports the number of bidirectional streams which can still be opened
for the QUIC connection. It depends directly to the flow-control value
advertised by the peer. Thus, this ensures that attach won't cause any
flow control violation.