Terminate recursive calls back to same scripted event listener

When we switched over to the listener invoker model for JIRA to support issue link events, we removed the code that handles the fact that the version synchroniser listener itself publishes version events and then gets called again, causing it to get stuck in a loop. The code we removed terminates the listener if it calls back on itself.

We need to add this code back. This is also needed for other types of events as well, for example worklog events.