race between CollectionCloner::_findCallback and CollectionCloner::_insertDocuments

While the collection cloner is inserting a single batch of documents into the database in _insertDocuments (before it reaches point where it calls into the collection bulk loader), the getMore command on the sync source may return a failure (for example, collection gets dropped) which terminates the collection cloner by way of the _finishCallback function. _finishCallback destroys the collection bulk loader which maybe trigger an invariant in _insertDocuments running a separate thread.