[ESI] Ditch Capnp and replace it with Protobuf/gRPC

[teqdruid]

I've had it struggling with capnp:

• It's obscurity makes it really hard to google for solutions.
• libkj -- which is required to use its RPC -- has a (let's say unique) API and async programming model. It makes a closed world assumption which isn't reasonable as it makes integration with other (standard) code pretty much impossible without jumping through perf-killing hoops. It also has horrible documentation and no "here's how you do things in bizarro land" guide.
• I don't need the perf it promises.

[teqdruid]

@mortbopet Do you think this is a good idea? I ran into another problem trying to combine C++ stdlib async calls with rpc.