index selected Strelka result fields

The analytics team (via @sephthir) has asked me to index:

* `strelka.file.tree.node`
* `strekla.file.tree.root`
* `strelka.file.tree.parent`

To allow them to build a tree of a given file scan. This issue tracks that request.

Indexing as:

* `filescan.tree.depth`
* `filescan.tree.node`
* `filescan.tree.parent`
* `filescan.tree.root`