Investigate moving to grpc

[dgryski]

[Dieterbe]

What are the benefits ?

[dgryski]

All the connections are pipelined over a single TCP connection, which means we don't have (literally) thousands of network connections to every server across a large cluster topology. That's the primary benefit I'd like. We'd probably also get a small improvement from the switch to protobuf v3. (Small in this case -- fewer pointers because v3 has values instead of making everything *Int32, etc.)

I'd probably add context and x/net/trace support while I was at it.

[nnuss]

+1

[dgryski]

Step 1 of moving to protobuf v3 is done.