transaction: allow registering a post-close callback The addchangegroup code considers the transaction done after a 'tr.close()' call and schedules the hook's execution for after lock release. In the nested transaction case, the transaction is not yet committed and we must delay this scheduling. We add an 'addpostclose' method (like the 'addpending' and 'addfinalize' ones) that registers code to be run if the transaction is successfully committed.