transaction: allow registering a temporary transaction file During the transaction, files may be created to store or expose data involved in the transaction (eg: changelog index data are written in a 'changelog.i.a' for hooks). But we do not have an official way to record such file creation and make sure they are cleaned up. The lack of clean-up is currently okay because there is a single file involved and a single producer/consumer. However, as we want to expose more data (bookmarks, phases, obsmarker) we need something more solid. The 'backupentries' mechanism could handle that. Temporary files can be encoded as a backup of nothing '('', <temporarypath>)'. We "need" to attach it to the same mechanism as we use to be able to use temporary transaction files outside of .'store/' and 'backupentries' is expected to gain such feature. This changeset makes it clear that we should rename 'backupentries' to something more generic.