BUG/MINOR: mux-quic: fix sedesc leak on BE side

BUG/MINOR: mux-quic: fix sedesc leak on BE side

On backend side, streams are instantiated prior to their QCS MUX
counterpart. Thus, QCS can reuse the stream-endpoint already allocated
with the streams, either on qmux_init() or attach operation.

However, a stream-endpoint is also always allocated in every qcs_new()
invokation. For backend QCS, it is thus overwritten on
qmux_init()/attach operation. This causes a memleak.

Fix this by restricting allocation of stream-endpoint only for frontend
connection.

This does not need to be backported.