Removed Connection::SetOpen.
Added ThreadChannel<void> specialization.
USB ACM connection will now flush before completing a disconnect.

This allows a shupito connected via a tunnel through another shupito to be
disconnected cleanly.

A new connection state, st_disconnecting, was added to support the
disconnect flush correctly.
Added proper flush to usbshupito23conn.
usbshupito23conn will auto-disconnect after name persist.