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.
This patch introduces a new "domaininfo" package, which implements a
database with information about domains. In particular, it tracks
incoming and outgoing security levels.
That information is used in incoming and outgoing SMTP to prevent
downgrades.
