Distribution should use its own threadpool of scheduled events

Sling distribution should use its own threadpool for scheduled events instead of the default threadpool.

affected classes:

• org.apache.sling.distribution.journal.impl.precondition.StagingPrecondition
• org.apache.sling.distribution.journal.queue.impl.PubQueueProviderImpl
• org.apache.sling.distribution.journal.impl.discovery.DiscoveryService
• org.apache.sling.distribution.packaging.impl.ResourceDistributionPackageCleanup

These scheduled jobs should use a dedicated thread-pool "org-apache-sling-distribution", for details see the documentation