Simplify fault and restart code

[mkeeter]

The conditions under which we restart a client IO task are somewhat baroque:

• It may halt spontaneously (e.g. upon a socket error)
• We may fault it explicitly (e.g. upon IO error). In this case, we want to skip all work associated with this client.
• We may restart the IO task if initial negotiation fails
• ...and when replacing a Downstairs
• ...and when deactivated

This PR adds more specific types: enum ClientNegotiationFailed and enum ClientFaultReason. Mid-negotiation restarts must provide a ClientNegotiationFailed; faulting the IO task must provide a ClientFaultReason. Both of these types are then converted into a ClientStopReason for logging.

In addition, faulting a client is now done through Downstairs::fault_client instead of calling both Downstairs::skip_all_jobs and DownstairsClient::fault. Having a single function makes this harder to mess up!