maintnerd/maintnerd: add --config=godata mode, to run using using godata For upcoming API development, permit running maintnerd in non-syncing (no new mutations logged) mode, using only the data from the godata package. Updates golang/go#19866 Updates golang/go#20222 Change-Id: I8c66c7f5e29b5702b481da0fec8f270f93f484a9 Reviewed-on: https://go-review.googlesource.com/43557 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>