Add HTTP method to middleware request object

[matiasinsaurralde]

This covers both coprocess & JSVM, related to #1219.

[buger]

I see you added the following comment:

There's a data structure that's based on this and it's used for Protocol Buffer support, make sure to update it on every change (coprocess/proto/coprocess_session_state.proto).

But what about coprocess/coprocess_session_state.pb.go, does it gets updated automatically?

If so, what commands I should run to do that?

[buger]

This stuff is new, did you used some experimental protobuf version to build this?

[matiasinsaurralde]

The Dispatcher class looks mostly the same, theBetaDispatcher changed a bit, I will investigate this (shouldn't affect).

[buger]

Now it required grpc packages (previously it dont).

I would say that it is better to run script with same protobuf version as we officially use now in CI.

[matiasinsaurralde]

I've updated my Protocol Buffers setup to make sure that we're using the latest stable version, so the bindings have been generated using grpcio-1.8.3 (installed via pip) and protobuf-3.5.1.
protoc returns the following:

$ protoc --version
libprotoc 3.5.1

I have also updated the instructions and the script that we use to generate the bindings to explicitly use Python 3 (previously it could mess things up if you ran the Python generator using "python" instead of "python3").
I will push the second PR that extends our definitions in a few minutes but would like to get this merged first so that we can rebase the other PR.