Network servers should not ignore ServiceError.

[ximon18]

This PR addresses an omission in the net::server::dgram and net::server::stream modules whereby they iterate over the stream of service responses accepting only Some(Ok) results and cease iterating on anything else. To stop on None is fine, but to stop on Err without taking any action is not.

Background: The Service interface was designed such that services can easily return Err(ServiceError) with the inteion that this allows them to easily produce certain standard DNS error responses without doing the work of constructing the response message correclty, keeping their error handling code paths simple. The ServiceError type has an rcode() method which the network server was supposed to use to construct the appopriate DNS response message, e.g. ServiceError::InternalError translates to rcode SERVFAIL.

But since the network servers never handle the service error case other than to ignore it that means in such cases the client wouldn't receive any DNS response message at all! This was not the intention, it was simply overlooked and clearly there also aren't any test cases that exercise this.

This PR makes three improvements:

1. Detect ServiceError and construct and return the appropriate DNS response error message to the client.
2. Do this once in common code, factor out such common code from the network servers to a new ServiceInvoker base trait with default fn impls, with some network server specific methods to be filled in by the network server impls.
3. Make the network server impls behave the same way for code that should already have been the same but through duplication had slight differences. E.g. till now the UDP server would spawn a background async task before even turning the message bytes into a Message object, while the TCP server would defer async task creation till a bit later.

This PR does NOT (yet?) address the lack of test cases for this logic.