Ignore events on destroyed objects

Destroyed objects no longer listen to firebase, so they don't expect to 
receive the update.
Fix serialising unsaved model with undefined associations

If unsaved & value is null/undefined & we don't have a snapshot then don't try 
and look for it.
Inflate fetched collections immediately.

Otherwise we can return a collection we have the data for but isn't 
inflated until the next runloop & calling code thinks it's empty.

Eg.

this.store.fetch("people").then(function(people){
  return people.get("firstObject") // null because not inflated yet
});
Fallback to 'true' for meta-models with no data

If we have a meta-model with attrs/relationships which are blank then
serialising would return {}, which is treated as empty in FB and
deletes the reference.
Adding items to indexed collection sets parent & wraps if necessary