Improve speed of Delete Project operation

The time taken to delete a project is proportional to the number of issues it has. The number of comments, attachments, issue links, etc each issue has also affects the deletion time.

As an Issue Deleted event needs to be thrown for each issue, and each issue and its comments need to be removed from the lucene index, the deletetion has to be performed one issue at a time.

Need to investigate whether we can speed this up.