Add service that calls client services

[externl]

We should have a service that makes calls on an established connection back to services defined on the client. This service shouldn't be too complicated (require too much setup on the part of the client) or too simple.

See also:

• https://zeroc.com/blog/other-way-around
• https://github.com/icerpc/icerpc-csharp/tree/0.3.x/examples/slice/Thermostat

Idea:

• Wakeup service: the client asks the server to "wake it up" after a prescribed amount of time. If the client service does not return an "awake" response (an enum Awake, Snooze), the server will keep trying.