Calling Onyx.clear() does not send the right data to collection callbacks using waitForCollectionCallback

[tgolen]

Problem

Since Expensify/react-native-onyx#214 was written, when Onyx.clear() is called, and a collection callback is using waitForCollectionCallback is being called n times with each item in the collection passed to it. This is bad because when an Onyx connection is using waitForCollectionCallback, the callback always expects to be called once with a single object for the entire collection.

This results in unexpected behavior for these callbacks when Onyx is cleared.

Solution

Update Onyx.clear() to properly handle this case and only trigger the callback once with the entire collection.

cc @yuwenmemon