When deregistering a service, consul also deregisters the associated
checks. The current state keeps track of all services and all checks
separately and deregisters them in sequence, which leads, whether during
syncs or shutdowns, to check deregistrations happening twice and failing
the second time (generating errors in logs)

This fix includes:
- a fix to the sync logic that just pulls the checks *after* the
services have been synced
- a fix to the shutdown mechanism that gets an updated list of checks
after deregistering the services, so that we get a cleaner check
deregistration process.