lib/db: Use explicit byte type for type prefixes (#6754)
lib/db: Use explicit byte type for type prefixes (#6754)

This makes Go 1.15 test/vet happy, avoiding "conversion from untyped int
to string yields a string of one rune" warning where we do
string(KeyTypeWhatever) in namespaced.go.

It also clarifies and enforces the currently allowed range of these
numbers so I think it's fine.
imsodin/syncthing,wweich/syncthing,imsodin/syncthing,imsodin/syncthing,syncthing/syncthing,wweich/syncthing,syncthing/syncthing,syncthing/syncthing,imsodin/syncthing,wweich/syncthing,imsodin/syncthing,syncthing/syncthing,syncthing/syncthing,syncthing/syncthing,wweich/syncthing