Rift should reset the index offset in the case of a rollback

We are not correctly adjusting the offset that the index stores in the case of a rollback. After a rollback, we should reset the indexes offset to be that of the size of the data store (similar to when the store is first opened). This is important because in the case of a resume (with a rollback) the index will continue writing assuming that the data offset hasn't changed. This will lead to a EOF error on a restore because we discard the incorrect amount of data during the FullScan.