MEDIUM: connection: inform si_alloc_conn() whether existing conn is OK or not

When allocating a new connection, only the caller knows whether it's
acceptable to reuse the previous one or not. Let's pass this information
to si_alloc_conn() which will do the cleanup if the connection is not
acceptable.
diff --git a/src/backend.c b/src/backend.c
index c680777..657da9d 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -982,7 +982,7 @@
 int connect_server(struct session *s)
 {
 	struct connection *cli_conn;
-	struct connection *srv_conn = si_alloc_conn(s->req->cons);
+	struct connection *srv_conn = si_alloc_conn(s->req->cons, 0);
 	struct server *srv;
 	int err;