Reduce the number of clone calls in the gRPC pseudo-node

[ipetr0v]

If I understand correctly - Hyper creates a new instance of a server for each user request.
More than that, it can pass a server generator to other threads that may work with individual user connections.
So we need a make_service_fn that creates an asynchronous server generator used by different threads, and a service_fn function that is used by a thread to generate a server instance to serve each user request (serve is also async).

In order to deal with the Rust borrow checker we need to clone the server 3 times:

oak/oak/server/rust/oak_runtime/src/node/grpc_server.rs

Lines 314 to 323 in 1d0e436

	let generator_server = server.clone();
	let service = hyper::service::make_service_fn(move |_| {
	let connection_server = generator_server.clone();
	async move {
	Ok::<_, hyper::Error>(hyper::service::service_fn(move |req| {
	let request_server = connection_server.clone();
	async move { request_server.serve(req).await }
	}))
	}
	});

Based on the Slack thread.