MINOR: mux-h2: add a trace context filling helper

MINOR: mux-h2: add a trace context filling helper

This helper is able to find a connection, a session, a stream, a
frontend or a backend from its args.

Note that this required to always make sure that h2s->sess is reset on
allocation because it's normally initialized later for backend streams,
and producing traces between the two could pre-fill a bad pointer in
the trace_ctx.