lib/model: Stop summary sender faster (ref #6319) (#6341)
lib/model: Stop summary sender faster (ref #6319) (#6341)

One of the causes of "panic: database is closed" is that we try to send
summaries after it's been closed. Calculating summaries can take a long
time and if we have a lot of folders it's not unreasonable to think
that we might be stopped in this loop, so prepare to bail here.

* push

imsodin/syncthing,syncthing/syncthing,wweich/syncthing,wweich/syncthing,imsodin/syncthing,syncthing/syncthing,imsodin/syncthing,syncthing/syncthing,syncthing/syncthing,wweich/syncthing,imsodin/syncthing,syncthing/syncthing,syncthing/syncthing,imsodin/syncthing,wweich/syncthing