OPENDJ-2580 File-based changelog growing and filling up the disk when using modrate

Configuring a server as RS only should disable the change indexer. Change-log purging is based on it, but if it disabled, it should purge on purge-delay only.
Change from testing only the indexer being enabled to also checking there actually is at least a domain for which external change-log is enabled, so that indexer had data to work on and can return a purge CSN.