Fix EZP-21599: content/publish transaction counter error

http://jira.ez.no/browse/EZP-21599

Alternative patch for https://github.com/ezsystems/ezpublish-legacy/pull/766.

Adds an 'on-interupt' key for the trigger definition, that accepts as an argument a method definition. The pre_publish trigger in content/publish defines commitTransaction as the callback method, committing the unfinished transaction.

Same description and reproduction steps as https://github.com/ezsystems/ezpublish-legacy/pull/766.