added (disabled) experimental Solr JOIN

This experimental Solr JOIN if for keeping permissions out
of the primary Solr documents and instead putting them in
new "permission" documents that are something of a sidecar
to each of the primary documents.

See also:

- http://lucene.472066.n3.nabble.com/Solr-JOIN-keeping-permission-data-out-of-primary-documents-td4169739.html
- https://gist.github.com/pdurbin/4d27fea7b431ef3bf4f9