Remove async registration of apiWatchers, since it breaks discovering `role: endpoints` and `role: endpointslices` targets,
which depend on pod and service objects.
There is no need in reloading `endpoints` and `endpointslices` targets if the referenced `pod` or `service` objects change,
since in this case the corresponding `endpoints` and `endpointslices` objects should also change because they contain
ResourceVersion of the referenced `pod` or `service` objects, which is modified on object update.
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1182
Previously multiple scrape jobs could create multiple watchers for the same apiURL. Now only a single watcher is used.
This should reduce load on Kubernetes API server when many scrape job configs use Kubernetes service discovery.
Serialize reloading per-role objects, so they don't occupy too much memory when objects for many scrape jobs are simultaneously refreshed.
Do not reload per-role objects if they were already refreshed by concurrent goroutines. This should reduce load on Kubernetes API server
when big number of scrape jobs are configured for the same Kubernetes role.
This is a follow-up for 17b87725ed
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1113
Previously vmagent was creating a separate Kubernetes object cache per each scrape job.
This could result in increased memory usage when monitoring a Kubernetes cluster with big number of objects (pods / nodes / services, etc.)
as seen at https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1113
Now it uses a shared map of scrape objects across multiple scrape jobs.
