This patch tidies how defaults are handled in the config, using a new
logic to allow "overriding" one config (the default) with another (the
user supplied).
It also improves how the comparisons are done in the tests, using the
more convenient "github.com/google/go-cmp/cmp" package, which also
prints nice diffs on errors.
This is in preparation for a future path where the override mechanism
will be reused.
There is a new protobuf library (and corresponding code generator) for
Go: google.golang.org/protobuf.
It is fairly compatible with the previous v1 API
(github.com/golang/protobuf), but there are some changes.
This patch adjusts the code and generated files to the new API.
The on-wire/on-disk format remains unchanged so this should be
transparent to the users.
