archive v1.12.0 tag as "old-v1.12.0"

This tag was created pre go modules, and was never intended to be a "release".
Before go modules, tags were "opt-in", and could "optionally" be used as a
reference for a version to use, but when go modules were introduced, tags
got "meaning", and in this case "latest" is no longer "latest", but using
the last "released" version (v1.12.0 in this case).

While there are no plans to start tagging releases of this project, we do have
the intent to move this repository, and to make it a module (adding a go.mod).

To prevent users from getting an old version of this code, and to run into
problems where the old version uses the incorrect import path, this tag is
created from the v1.12.0 tag (commit 9d4c2f7),
with the intent to delete that tag.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Tag version of swarmkit shipped in Docker 1.12.0

Merge pull request #464 from stevvooe/fix-protobuf-generate

api: consolidate protobuf types into a single package

Merge pull request #165 from aaronlehmann/isleader-atomicity

raft: Use IsLeader method to avoid race detector complaint