all: Become a Go module (fixes #5148) (#5384)

* go mod init; rm -rf vendor

* tweak proto files and generation

* go mod vendor

* clean up build.go

* protobuf literals in tests

* downgrade gogo/protobuf

vendor/github.com/petermattis/goid/.gitignore

@@ -0,0 +1,4 @@
+*~
+*.test
+.*.swp
+.DS_Store

vendor/github.com/petermattis/goid/.travis.yml

@@ -0,0 +1,10 @@
+language: go
+
+go:
+  - 1.3.x
+  - 1.4.x
+  - 1.5.x
+  - 1.6.x
+  - 1.7.x
+  - 1.8.x
+  - master

vendor/github.com/petermattis/goid/README.md

@@ -0,0 +1,4 @@
+# goid [![Build Status](https://travis-ci.org/petermattis/goid.svg?branch=master)](https://travis-ci.org/petermattis/goid)
+
+Programatically retrieve the current goroutine's ID. See [the CI
+configuration](.travis.yml) for supported Go versions.