MXS-874: Clear closed state before reconnecting to a server

The backend reference states should be cleared when a reconnection attempt
is made. Should the creation of a new DCB succeed, the backend should no
longer be closed.

server/modules/routing/readwritesplit/readwritesplit.c

@@ -2889,6 +2889,8 @@ bool connect_server(backend_ref_t *bref, SESSION *session, bool execute_history)
 
     if (bref->bref_dcb != NULL)
     {
+        bref_clear_state(bref, BREF_CLOSED);
+
         if (!execute_history || execute_sescmd_history(bref))
         {
             /** Add a callback for unresponsive server */