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>
2 files changed