MEDIUM: stream: don't rely on the session's listener anymore in stream_new()

MEDIUM: stream: don't rely on the session's listener anymore in stream_new()

When the stream is instanciated from an applet, it doesn't necessarily
have a listener. The listener was sparsely used there, just to retrieve
the task function, update the listeners' stats, and set the analysers
and default target, both of which are often zero from applets. Thus
these elements are now initialized with default values that the caller
is free to change if desired.