support unregistering a service

[weissi]

In certain cases it could be useful to register and then unregister a service, maybe because that service has been shut down independently for some reason.

Today, this is possible but just forgetting about a registered service (and making the shutdown function do nothing). Unfortunately, that would cause the registration to consume memory forever...

I'd like to see an API like this:

// the return value is usually ignore (`@discardableResult`)
let registrationToken = lifecycle.register(...)
...
lifecycle.unregister(registrationToken, runShutdown: false)

[weissi]

CC @Davidde94

[tomerd]

closed via #121