PAC-386 - Updating MongoDao to ensure closure of all DbCursors

- Replacing findOne with find.limit(1) to be able to handle cursor closure
- Fixing WriteConcern to using MongoDbOptions not overriding it at call time
Merge pull request #3 from bazaarvoice/mgoodnow-fixingWriteConcern

PAC-386 - Updating MongoDao to ensure closure of all DbCursors