MINOR: stream: provide a few helpers to retrieve frontend, listener and origin

Expressions are quite long when using strm_sess(strm)->whatever, so let's
provide a few helpers : strm_fe(), strm_li(), strm_orig().
diff --git a/src/compression.c b/src/compression.c
index 814c24f..dc5ad5a 100644
--- a/src/compression.c
+++ b/src/compression.c
@@ -319,10 +319,10 @@
 	/* update input rate */
 	if (s->comp_ctx && s->comp_ctx->cur_lvl > 0) {
 		update_freq_ctr(&global.comp_bps_in, msg->next);
-		strm_sess(s)->fe->fe_counters.comp_in += msg->next;
+		strm_fe(s)->fe_counters.comp_in += msg->next;
 		s->be->be_counters.comp_in += msg->next;
 	} else {
-		strm_sess(s)->fe->fe_counters.comp_byp += msg->next;
+		strm_fe(s)->fe_counters.comp_byp += msg->next;
 		s->be->be_counters.comp_byp += msg->next;
 	}
 
@@ -346,7 +346,7 @@
 
 	if (s->comp_ctx && s->comp_ctx->cur_lvl > 0) {
 		update_freq_ctr(&global.comp_bps_out, to_forward);
-		strm_sess(s)->fe->fe_counters.comp_out += to_forward;
+		strm_fe(s)->fe_counters.comp_out += to_forward;
 		s->be->be_counters.comp_out += to_forward;
 	}