Pick servers that can be connected to as candidates

Only servers that qualify to be connected should be considered as
candidate servers. This triggered a debug assertion when a slave server
failed to execute a session command but it was chosen as a candidate
server later on.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -202,6 +202,11 @@ private:
                m_retry_duration < m_config.delayed_retry_timeout &&
                !session_trx_is_active(m_client->session);
     }
+
+    inline bool can_recover_servers() const
+    {
+        return !m_config.disable_sescmd_history || m_recv_sescmd == 0;
+    }
 };
 
 /**