This is perhaps a better solution than b8551eed7e
and 8d0a655608 and 7942897d93. It is certainly less
fragile.
It is more consistent with other plugin standards,
where modules are closed with the last instance in a session.
Then again keeping the VST3 factory around is beneficial
when switching snapshots.
Discuss, and let's watch for issues when re-loading a
previously unloaded VST3 module.
cd5369c7cd