Provide a way to set the JcrRepository descriptors from the JcrConfiguration and JcrEngine

The JcrRepository constructor takes a Map for the descriptors, and a few of these are checked by the TCK tests. It would be nice to control the Repository.QUERY_XPATH_DOC_ORDER and Repository.QUERY_XPATH_POS_INDEX descriptors via JcrConfiguration and the configuration file to directly control certain XPath tests.

To date we've gotten around it by limiting the number of nodes.