CB for connect-store, so mongo-store will only cb upon conn complete

Following the example provided here: http://stackoverflow.com/a/22698992/3440368

For this issue : https://github.com/balderdashy/sails/issues/1959

I'm not sure if this is the right fix - the tests pass, but would appreciate if a developer could guide me through :) (Namely, I'm not sure how to write a test for this.)