~Session calls AudioEngine::remove_session(), which fades out,
then unsets the session, and then in libs/ardour/audioengine.cc:460
```
session_removed.signal(); // wakes up thread that initiated session removal
```
Session d'tor continues, and calls Port::PortDrop(), which
unregisters all ports one at a time.
Concurrently, AudioEngine continues, and calls PortManager::silence_outputs
which gets all ports first (!), and then iterates over them.
There is a race condition which can lead to
DummyAudioBackend::get_buffer: Assertion `valid_port (port)' failed
95de61f74e