BUG/MEDIUM: mux-quic: Create sedesc in same time of the QUIC stream

BUG/MEDIUM: mux-quic: Create sedesc in same time of the QUIC stream

Recent changes to save abort reason revealed an issue during the QUIC stream
creation. Indeed, by design, when a mux stream is created, it must always
have a valid stream-endpoint descriptor and it must remain valid till the
mux stream destruction. On frontend side, it is the multiplexer
responsibility to create it and set it as orphan. On the backend side, the
sedesc is provided by the upper layer. It is the sedesc of the back
stream-connector.

For the QUIC multiplexer, the stream-endpoint descriptor was only created
when the stream-connector was created and attached on it. It is unexpected
and some bugs may be introduced because there is no valid sedesc on a QUIC
stream. And a recent bug was introduced for this reason.

This patch must be backported as far as 2.6.