Revisit the asynchronous ipc implementation

Seems the current approach of implementing AsyncTable is in trouble as we expose some netty classes in our public interfaces.

I agree that we should not do this. The AsyncTable should be implemented using the asynchronous protobuf stub, and we could use CompletableFuture or Deferred instead of netty's Future. I think the problem of netty's future is that it is tighten with netty's executor. This makes it impossible to use without netty.