This works around a race-condition, calling d'tors from
two threads concurrently.
The GUI thread destroys ctrl surfaces. ~~MIDIControllable()
calls ::drop_external_control() -> ::midi_forget()
This unsubscribes from signals (notably MIDI::Parser events)
by calling ScopedConnection::disconnect(), Connection::disconnect().
At the same time auto_connect_thread can call
PortManager::clear_pending_port_deletions() which removes
the MIDI port and destorys the MIDI::Parser.
~Parser() calls Connection::signal_going_away() to invalidate
connected signals.
This can deadlock if it is called concurrently with
Connection::disconnect() on the same signal.
see also
https://discourse.ardour.org/t/ardour-session-close-hangs/106523/10
ee8e8da614