Multi-container: Dispatch incoming requests to appropriate executor

[eddyashton]

For initial implementation (with HTTP transport), we can simply iterate through all registered executors, and choose the first match. Additionally, we can queue all client requests and actually provide them on polls from each executor, if that's easier for now. With gRPC, we should be able to dispatch and queue as each request comes in.

Eventually I think we should use r3 for this, though I haven't worked out exactly how/when we compile the prefix trie.