hook: add a generic hook right before we commit a transaction We are adding a 'txnclose' hook that will be run right before a transaction is closed. Hooks running at that time will have access to the full transaction content through both 'hookargs' content and on-disk reading. They will be able to abort the transaction.

transaction: add a validation stage The 'transaction' object can now be fed a 'validator' function. This function will be run right before the transaction is closed to validate its content. The target usage is hooks. The validation function is expected to raise an exception when it wants to abort the transaction. This only introduce the idea with a default no-op validator. Actual usage is in the next changeset.

hook: add a generic hook after transaction has been closed We are adding generic hooking for all transactions. We may have useful information about what happened during the transaction, user of the transaction should have filled the 'hookargs' dictionnary of the transaction. This hook is simple because it has no power to rollback the transaction.

hook: have a generic hook for transaction opening We are adding generic hooking for all transactions. We do not really have any useful information to include when opening the transaction but this is a useful time to allow a hook anyway. We better let people abort transaction before they happen than after multiple seconds/minutes of processing.