New branch for removing gogo/protobuf

I've been working on upgrading protobuf packages in containerd. Now Protobuild supports new protoc-gen-go and #96 will add new protoc-gen-go-ttrpc which should be able to generate ttrpc code that works with the new protoc-gen-go.

However, next few PRs will make some backward-incompatible changes, namely

- Removing gogo/protobuf from ttrpc itself
- Updating example/

Since ttrpc is used by containerd which has relatively long support policy, we would make some enhancements such as #75 and #94. So I'm reluctant to make ttrpc's main branch incompatible with gogo/protobuf right now.

Instead, could we have a branch to remove gogo/protobuf? If so, do we want to do Go-style branching (e.g. `v2`) instead of containerd-style (`release/1.0`)?