XPackPlugin holds data in statics and can only be initialized once. This
caused tests to fail primarily when running with a low max-workers.
Replaced usages with the LocalStateCompositeXPackPlugin, which handles
this properly for testing.
This commit merges the `object-fields` feature branch. The new 'flattened
object' field type allows an entire JSON object to be indexed into a field, and
provides limited search functionality over the field's contents.