Unintended side effect in our cancel functions (we may unintentionally remove observers attached via other means)

[katowulf]

Both firebase_list_factory.js and firebase_object_factory.js return a cancel function which internally calls ref.off(); However, since no arguments are provided, this has the side effect of terminating any other listeners attached to that database path and could lead to some unexpected and hard to debug behaviors.

We should change this to include the event and callback function in the calls to off().