Simple broker sends notice after disconnect

Before this change the simple broker simply removed subscriptions
upon receiving a DISCONNECT message assuming it was a result of
a client STOMP WebSocket session ending.

However, if the server-side application sends a DISCONNECT to
the broker in order to terminate a session, the STOMP WebSocket
session could remain unware without any further action. This
change ensures the simple broker sends a DISCONNECT_ACK message
downstream whenever it receives a DISCONNECT.

Issue: SPR-12288