jsonpb: avoid copying string-valued map-keys

[dhowden]

Encoding large maps with string keys is very expensive. Profiling indicated that the culprit was github.com/golang/protobuf/jsonpb.mapKeys.Less which is used to ensure that the ordering of the map keys are consistent since Go map iteration is random.

By adding a string-specific case we avoid the overhead of going through the fmt.Sprintf calls, which removes a lot of unnecessary allocations.

I suspect that this can be taken further - the requirement of ordered keys does not apply to the JSON rep, so I believe it could be be removed in this case (https://developers.google.com/protocol-buffers/docs/proto3#json). Note: the "text format" rep does require ordering, so maybe this is why the JSON code inherited it...

[puellanivis]

👍

[dhowden]

Build is passing for go versions 1.10.x and 1.6.x, but failing on 1.x due to:

go build ./protoc-gen-go/testdata/grpc/grpc.pb.go
# golang.org/x/net/http2
../../../golang.org/x/net/http2/go111.go:12:30: trace.WroteHeaderField undefined (type *clientTrace has no field or method WroteHeaderField)
../../../golang.org/x/net/http2/go111.go:16:26: trace.WroteHeaderField undefined (type *clientTrace has no field or method WroteHeaderField)
../../../golang.org/x/net/http2/go111.go:17:8: trace.WroteHeaderField undefined (type *clientTrace has no field or method WroteHeaderField)
make: *** [test] Error 2

https://travis-ci.org/golang/protobuf/jobs/403362012#L634

[puellanivis]

This is a known break in the travis CI, and not at all the result of this patch.

[dsnet]

Thank you for the PR. @puellanivis is correct that this failure is known (see #650 (comment)).