[MAJOR] session-counters: split FE and BE track counters

Having a single tracking pointer for both frontend and backend counters
does not work. Instead let's have one for each. The keyword has changed
to "track-be-counters" and "track-fe-counters", and the ACL "trk_*"
changed to "trkfe_*" and "trkbe_*".
diff --git a/src/proto_http.c b/src/proto_http.c
index 7e28762..1d126fc 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -3704,6 +3704,7 @@
 
 	s->logs.t_close = tv_ms_elapsed(&s->logs.tv_accept, &now);
 	session_process_counters(s);
+	session_stop_backend_counters(s);
 
 	if (s->txn.status) {
 		int n;