maintner: move record reading and formatting code to its own package

It's starting to pollute the package, and seems separable. I also plan
to use it more from elsewhere in upcoming CLs.

Updates golang/go#19866

Change-Id: I7b2add37f74ed42c2f78939924f19d8322179823
Reviewed-on: https://go-review.googlesource.com/40868
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed