MAJOR: sample: pass a pointer to the session to each sample fetch function

Many such function need a session, and till now they used to dereference
the stream. Once we remove the stream from the embryonic session, this
will not be possible anymore.

So as of now, sample fetch functions will be called with this :

   - sess = NULL,  strm = NULL                     : never
   - sess = valid, strm = NULL                     : tcp-req connection
   - sess = valid, strm = valid, strm->txn = NULL  : tcp-req content
   - sess = valid, strm = valid, strm->txn = valid : http-req / http-res
diff --git a/include/proto/stream.h b/include/proto/stream.h
index 4d23470..1cd506a 100644
--- a/include/proto/stream.h
+++ b/include/proto/stream.h
@@ -48,7 +48,7 @@
 void sess_change_server(struct stream *sess, struct server *newsrv);
 struct task *process_stream(struct task *t);
 void default_srv_error(struct stream *s, struct stream_interface *si);
-struct stkctr *smp_fetch_sc_stkctr(struct stream *l4, const struct arg *args, const char *kw);
+struct stkctr *smp_fetch_sc_stkctr(struct session *sess, struct stream *strm, const struct arg *args, const char *kw);
 int parse_track_counters(char **args, int *arg,
 			 int section_type, struct proxy *curpx,
 			 struct track_ctr_prm *prm,