This changes the BEP protocol to use protocol buffer serialization
instead of XDR, and therefore also the database format. The local
discovery protocol is also updated to be protocol buffer format.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3276
LGTM: AudriusButkevicius
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
