This fixes the deadlock by reducing where we hold the various locks. To
start with it splits up the existing "mut" into a "listenersMut" and a
"curConMut" as these are the two things being protected and I can see no
relation between them that requires a shared lock. It also moves all
model calls outside of the lock, as I see no reason to hold the lock
while calling the model (and it's risky, as proven).
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3069