maintner: flesh out godata package, add caching network mutation source

Also, the maintner.Corpus constructor is now gone. An API is added for
maintnerd to become the leader and specify the cache dir.

Updates golang/go#19866

Change-Id: Ia726aa00ca1337b6c130cfee040ff9a1f935d0c2
Reviewed-on: https://go-review.googlesource.com/42148
Reviewed-by: Kevin Burke <kev@inburke.com>
8 files changed