Adding cleanup of dibs transactions

[bombycilla]

Solves the issue in lighthouse #1924.

[kasperg]

Thank you for the contribution!

Before we merge this there are a few things I would like to see changed. I've hightligted some below.

In general remember to check whether the new/updated code follows the Drupal Coding Standards as required by the Ding Code Guideline. A growing number of editors and IDE allow integration with the Drupal Code Sniffer which makes checking quite easy.

[kasperg]

The referenced ticket provides a more elaborate reasoning behind why this cleanup is necessary. I would like to have this documented in the code comments as well.

If this is a general problem consider submitting this as a patch for the DIBS module instead of implementing it here.

[mikl]

I'm not sure I see the need for this change. Destroying financial transaction data should not be done lightly.

Even if we want this change in core, I think running it every time cron runs is a bit wasteful. It is a fairly expensive query, and it should be sufficient to run it every 24 hours.