Modest simplification refactoring (issue #10457).
Also, fixes issue #11538, which was caused in part by a hole in the vestigial two-phase loading.

Conversion to one-phase tile loading removed any need to track them separately.

Prevents querying a FeatureIndex built against a separate set of data, which can lead to invalid index exceptions.
The GeometryTileWorker 'data' member can still change independently of the data in the feature index, at the time 'setData' is called. The GeometryTileWorker maintains ownership of its local data (which may be used to re-parse) and clones the data for use by the FeatureIndex in the foreground.