Consider closing MRCP port when SIP call ends

Currently, we are not closing MRCP connections and we are leaving this to be done by the client.
This is because MRCP messages for a session can be transmitted over multiple TCP connections.
However, in practice I think this will not happen and so we should close the MRCP connection if the SIP call ends.
This will at least avoid leaving sockets around that be causing issues during load tests.