PoC: client channels

[bsideup]

This PR (based off #3987) proposes a potential implementation for Client Channels that allow server call services on connected clients.

The current implementation uses a bi-directional call as a transport by implementing an in-memory Netty channel due to the complexity of the underlying representation. A custom Marshaller is used for zero copy operations, to prevent additional buffers from being created. That is achieved with a custom Drainable implementation and a HasByteBuffer consumer.

The final version should of course be using a custom implementation of both ManagedChannel and InternalServer.

See grpc/grpc#14101 for the main discussion.

[linux-foundation-easycla]

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[bsideup]

Quick Update:

I was able to connect a grpc-go based client (exposing service) to grpc-java Netty-based server that successfully called a service defined on the client:
https://gist.github.com/bsideup/49a031c3360a68e4542e5597669fe51d