When we disable access to system indices, plugins will still need
a way to erase their state. The obvious and most pressing use
case for this is in tests, which need to be able to clean up the
state of a cluster in between groups of tests.

• Use a HandledTransportAction for reset action

My initial cut used a TransportMasterNodeAction, which requires code
that carefully manipulates cluster state. At least for the first cut and
testing, it seems like it will be much easier to use a client within a
HandledTransportAction, which effectively makes the
TransportResetFeatureStateAction a class that dispatches other transport
actions to do the real work.

• Clean up code by using a GroupedActionListener

• ML feature state cleaner

• Implement Transform feature state reset

• Change _features/reset path to _features/_reset

Out of an abundance of caution, I think the "reset" part of this path
should have a leading underscore, so that if there's ever a reason to
implement "GET _features/<feature_id>" we won't have to worry about
distinguishing "reset" from a feature name.

Co-authored-by: Gordon Brown gordon.brown@elastic.co
Co-authored-by: Elastic Machine elasticmachine@users.noreply.github.com

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