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.