MEDIUM: stream-int: queue idle connections at the server

Now we get a per-server list of all idle connections. That way we'll
be able to reclaim them upon shortage later.
diff --git a/src/backend.c b/src/backend.c
index f8185d7..bbe9573 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1052,6 +1052,11 @@
 
 	if (!reuse)
 		srv_conn = si_alloc_conn(&s->si[1]);
+	else {
+		/* reusing our connection, take it out of the idle list */
+		LIST_DEL(&srv_conn->list);
+		LIST_INIT(&srv_conn->list);
+	}
 
 	if (!srv_conn)
 		return SF_ERR_RESOURCE;