Function for UNICAST3 to remove connections in CLOSING state

When a connection is closed in UNICAST3, it is not actually destroyed and removed, but transitioned into the CLOSING state.

Only after conn_close_timeout ms is the connection really destroyed and removed.

This means that if a connection to P has state, e.g. seqno=25, is subsequently closed but we send another message to P before the connection is destroyed, seqno=26 will be sent. If the connection was destroyed before, then a new connection with seqno=1 would be used.

If an application wanted to destroy a connection to a peer, then this proposed function would do this.

The implementation will probably be a managed operation.