Document selection string tester

In a given content cluster, using the schema, let the user easily create a selection string.

Then, add ability to run a visit using this, outputting how far into document space it is and how many documents are returned so far. This simplifies the process to develop a selection string and test it before deploying it - i.e. how many docs / fraction is kept when running the visit with the selection

visit can dump IDs only to speed up