The old usage pattern was to create a Walker with a bunch of attributes,
then call Walk() on it and nothing else. This extracts the attributes
into a Config struct and exposes a Walk(cfg Config) method instead, as
there was no reason to expose the state-holding walker type.
Also creates a few no-op implementations of the necessary interfaces
so that we can skip nil checks and simiplify things here and there.
Definitely look at this diff without whitespace.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3060
