Improve logging during AbstractSlingRepositoryManager.executeRepositoryInitializers()

As long running SlingRepositoryInitializer services can defer the startup quite a lot, it would be helpful to emit regular log messages (with INFO level) while looping through https://github.com/apache/sling-org-apache-sling-jcr-base/blob/c8a53dff4ec163e19a014c470126eb4450f78cde/src/main/java/org/apache/sling/jcr/base/AbstractSlingRepositoryManager.java#L625.

The log messages should include:

• Time since repo start,
• Currently running initializer (class name)
• Index of current initializer and total number of initializers

That should allow to predict by looking at the log, if there is any progress in starting the repo and how long it will probably still take.