Handle firebase sending `child_added` events before `value`

Firebase sends `value` last, so the initial `child_added` events trigger
updates of attributes & associations before the snapshot has been set.

Instead of going to the underlying _snapshot, we use the MutableSnapshot
which has the data set piecemeal before being set in `value`.

This bug was masked by the previous bug which trampled over nested queue entries, so 
we sometimes weren’t handling the `child_added` events before `value`
build v0.0.14