api: Use gogo/protobuf

[iwaseyusuke]

This patch fixes to migrate from protoc-gen-go to protoc-gen-gogofast.

Please note that with protoc-gen-gogo*, we need specify "M" parameters to override the well known types (e.g. any.Any of "google/protobuf").

Example of protoc command:

$ go get -u -v github.com/gogo/protobuf/protoc-gen-gogofast
$ export GOBGP=${GOPATH}/src/github.com/osrg/gobgp
$ protoc \
    -I=${GOPATH}/src/github.com/gogo/protobuf/protobuf \
    -I=${GOBGP}/api \
    --gogofast_out=plugins=grpc,Mgoogle/protobuf/any.proto=github.com/gogo/protobuf/types:${GOBGP}/api \
    ${GOBGP}/api/gobgp.proto \
    ${GOBGP}/api/attribute.proto

[fenollp]

I believe gogoproto died after Go's v2 of protobuf. This is alive though may be too binary-incompatible: https://github.com/planetscale/vtprotobuf