BUG/MEDIUM: backend: prefer-last-server breaks redispatch

Since 1.5-dev20, we have a working server-side keep-alive and an option
"prefer-last-server" to indicate that we explicitly want to reuse the
same server as the last one. Unfortunately this breaks the redispatch
feature because assign_server() insists on reusing the same server as
the first one attempted even if the connection failed to establish.

A simple solution consists in only considering the last connection if
it was connected. Otherwise there is no reason for being interested in
reusing the same server.

src/backend.c

@@ -543,6 +543,7 @@ int assign_server(struct session *s)
 	conn = objt_conn(s->req->cons->end);
 
 	if (conn &&
+	    (conn->flags & CO_FL_CONNECTED) &&
 	    ((s->be->options & PR_O_PREF_LAST) || (s->txn.flags & TX_PREFER_LAST)) &&
 	    objt_server(conn->target) && __objt_server(conn->target)->proxy == s->be &&
 	    srv_is_usable(__objt_server(conn->target)->state, __objt_server(conn->target)->eweight)) {