transaction: pass the transaction to 'postclose' callback This mirrors the API for 'pending' and 'finalize' callbacks. I do not have immediate usage planned for it, but I'm sure some callback will be happy to access transaction related data.

transaction: pass the transaction to 'finalize' callback The callback will likely need to perform some operation related to the transaction (eg: registering file update). So we better pass the current transaction as the callback argument. Otherwise callback that needs it has to rely on horrible weak reference trick. This allow already allow us to slay a wild weak reference usage.

transaction: pass the transaction to 'pending' callback The callback will likely need to perform some operation related to the transaction (eg: backing files up). So we better pass the current transaction as the callback argument. Otherwise callback that needs it has to rely on horrible weak reference trick. The first foreseen user of this is changelog._writepending. We would like it to register the temporary file it create for cleanup purpose.