Add index statistics helpers to system routines

Including exposing delete, which the service supported but wasn't externally available, and removing the JMX versions.

Also convert the `test-histograms.yaml` test to an IT, and corresponding resource files, as the yaml nested in strings nested in yaml didn't seem worth keeping/fixing.

The JMXInterpreterIT also depended on the index stats methods for no reason other than convenience. Add an internal bean to use to make it isolated.

Lastly, some minor shuffling and test helpers.