* nsd: block on removal of services
This PR uses a WaitGroup to ensure workload removals are complete
before returning from ServiceRegistrationHandler.RemoveWorkload of
the nomad service provider. The de-registration of individual services
still occurs asynchrously, but we must block on the parent removal
call so that we do not race with further operations on the same set
of services - e.g. in the case of a task restart where we de-register
and then re-register the services in quick succession.
Fixes#15032
* nsd: add e2e test for initial failing check and restart
