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/oschwald/maxminddb-golang/appveyor.yml

@@ -0,0 +1,19 @@
+version: "{build}"
+
+os: Windows Server 2012 R2
+
+clone_folder: c:\gopath\src\github.com\oschwald\maxminddb-golang
+
+environment:
+  GOPATH: c:\gopath
+
+install:
+  - echo %PATH%
+  - echo %GOPATH%
+  - git submodule update --init --recursive
+  - go version
+  - go env
+  - go get -v -t ./...
+
+build_script:
+  - go test -v ./...