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/acl.h b/include/proto/acl.h
index 7cadfce..6b94296 100644
--- a/include/proto/acl.h
+++ b/include/proto/acl.h
@@ -99,7 +99,7 @@
  * function only computes the condition, it does not apply the polarity required
  * by IF/UNLESS, it's up to the caller to do this.
  */
-enum acl_test_res acl_exec_cond(struct acl_cond *cond, struct proxy *px, struct stream *strm, unsigned int opt);
+enum acl_test_res acl_exec_cond(struct acl_cond *cond, struct proxy *px, struct session *sess, struct stream *strm, unsigned int opt);
 
 /* Returns a pointer to the first ACL conflicting with usage at place <where>
  * which is one of the SMP_VAL_* bits indicating a check place, or NULL if