Add sql Streaming Expression

The existing jdbc Streaming Expression is really designed for querying outside SQL engines.

The sql expression is designed to talk directly with Solr's parallel SQL interface.

This will make it much easier to intermingle SQL queries with other expressions and leverage the new math expressions on top of SQL result sets.

Sample syntax:

sql(dbcollection, stmt="SELECT ...")

The dbcollection parameter is the distributed front end to the database. Like all parallel SQL queries the stmt can reference any collection in the Zk instance.