In some cases, EMQX may decide to close socket and mark connection
at 'disconnected' state, for example, when DISCONNECTE packet is
received, or, when failed to write data to socket.
However, by the time EMQX decided to close the socket, the socket
might have already been closed by peer, and the `tcp_closed` envet
is already delivered to the process mailbox -- causing EMQX to
handle sock_close event at 'disconnected' state.
According to the Stomp v1.2 specification:
> The server MAY send ERROR frames if something goes wrong. In this case,
> it MUST then close the connection just after sending the ERROR frame
Additional, fixes the `is_superuser` is not working for all gateways
Zaiming (Stone) Shi