MINOR: counters: make it easier to extend the amount of tracked counters

By properly affecting the flags and values, it becomes easier to add
more tracked counters, for example for experimentation. It also slightly
reduces the code and the number of tests. No counters were added with
this patch.
diff --git a/src/peers.c b/src/peers.c
index 5e4ff35..61fb938 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1178,8 +1178,7 @@
 
 	/* init store persistence */
 	s->store_count = 0;
-	s->stkctr[0].entry = NULL;
-	s->stkctr[1].entry = NULL;
+	memset(s->stkctr, 0, sizeof(s->stkctr));
 
 	/* FIXME: the logs are horribly complicated now, because they are
 	 * defined in <p>, <p>, and later <be> and <be>.