protobuf: use protobuild to build protobufs

[stevvooe]

While the mess of generate.go files are working, they are also my fault. Hopefully, I will redeem this problem by providing https://github.com/stevvooe/protobuild for building protobufs.

This tool was created when making containerd to build its protobufs and others were interested in using something similar.

Integration will be similar to containerd/containerd#1187. If someone wants to pick this up, it would be great to have the documentation feedback, but I'll take it on if no one bites.

[tossmilestone]

I want to working on it. But the protobuild has a limitation on sub config in packages, and swarmkit has different protobuf configs on different packages.

Looking forward to add the feature.

[stevvooe]

@tossmilestone I don't think we'll be adding that to protobuild. SwarmKit has some subconfiguration, but we should be able to merge that into a single configuration. In places where there are conflicts, we should fix them.

[tossmilestone]

@stevvooe I have opened a pull request #2337 to do this.

[stevvooe]

@tossmilestone Thanks for the contribution!