Added support for fully instantiated node-MongoDB-native object use in lieu of a new connection 

This enhancement allows existing database connections to be reused for mongo sessions storage rather than requiring a separate connection be setup (which in any node app already using MongoDB, forces the sessions collection into a separate database, as node-mongo-native doesn't permit multiple opens on the same database by the same application. This enhancement adds flexibility of the mongo session handler overall, and extends it's suitability to a new set of use cases.