Support subqueries across reports and facts (and catalogs?)

For an upcoming PE release, wagner is planning on supporting the ability to filter events in the event inspector by fact values on the corresponding nodes.

The implementation of this is probably going to rely on the ability to express an `events` query in PuppetDB that has a subquery to join it against facts. I assume this will end up looking much like our current support for subqueries across facts and resources, similar to http://docs.puppetlabs.com/puppetdb/1.5/api/query/v3/operators.html#subquery-operators .

We may find that it's not much extra work to allow subqueries across catalogs at the same time, so that should probably be considered.

I believe that this functionality can't be implemented until we have support for historical facts (and perhaps catalogs) as well--or at least, that if we implemented it before we have historical facts, the implementation would change somewhat significantly after we do have historical facts. But I haven't put enough thought into it to fully flesh out the details.