Determine how _nest_path_ should be analyzed to support various use-cases

We know we need _nest_path_ in the schema for the new nested documents support, and we loosely know what goes in it. From a DocValues perspective, we've got it down; though we might tweak it. From an indexing (text analysis) perspective, we're not quite sure yet, though we've got a test schema, schema-nest.xml with a decent shot at it. Ultimately, how we index it will depend on the query/filter use-cases we need to support. So we'll review some of them here.

TBD: Not sure if the outcome of this task is just a "decide" or wether we also potentially add a few tests for some of these cases, and/or if we also add a FieldType to make declaring it as easy as a one-liner. A FieldType would have other benefits too once we're ready to make querying on the path easier.