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.
