ForestDB needs to provide rollback API with a facility for callback

For applications like ep-engine, the items that are rolled back on disk also needs to be rolled back in memory too. In order to facilitate that ForestDB needs to provide a rollback API with a document's target state. For example, if the document being rolled back was a new document, its target state should be deleted. If it was an updated version of the document, its target state should be the previous version. This way, the application can easily restore state of the rolled back documents.