Yarpc v2 apis which provides compatibility with golang/protobuf v2.

[gandhikrishna]

• [ ] Description and context for reviewers: one partner, one stranger
• [ ] Docs (package doc)
• [ ] Entry in CHANGELOG.md

Gogo is today used in yarpc as the core library for proto code generation and for proto runtime methods. This pull request aims to make yarpc compatible with golang/protobuf v2 APIs.

This PR introduces a new yarpc protoc plugin protoc-gen-yarpc-go-v2 which generates yarpc protobuf stubs using golang/protobuf v2 apis. Also, new yarpc v2 APIs which make use of golang/protobuf's for proto runtime methods.

[jronak]

@gandhikrishna, this is a very critical and huge diff with 3.5K+ LOC. I suggest you split the diff and stack this into smaller diffs for easier reviews. Logically the order of diffs can be encoding/v2/error -> encoding/v2/marshal -> ...

[biosvs]

Code migrated in separated diffs.