MEDIUM: conn-stream: Be able to pass endpoint to create a conn-stream

MEDIUM: conn-stream: Be able to pass endpoint to create a conn-stream

It is a transient commit to prepare next changes. It is possible to pass a
pre-allocated endpoint to create a new conn-stream. If it is NULL, a new
endpoint is created, otherwise the existing one is used. There no more
change at the conn-stream level.

In the applets, all conn-stream are created with no pre-allocated
endpoint. But for multiplexers, an endpoint is systematically created before
creating the conn-stream.