Migrate to gRPC

[Civil]

This is a pre-requirement for various pending improvements.

Examples of stuff we'll acheive:

• Better query multiplexing. gRPC uses HTTP/2 and because of that even when we have single requests, we won't have overhead for establishing/closing TCP connections.
• Better tracing support. This is pre-requirement for OpenTracing support
• Better Context Propagation - at this moment we need to marshal/unmarshal all interesting fields and send them as HTTP Headers, gRPC can do that for us.