Adding 'validateTree' field to the TreeMapper class. Valid options are
'ALL', 'POST', or 'NONE'. Default value is 'NONE'. Controls whether
to perform validation on the pages stored in the external database.
At the moment validaiton is only performed when the concurrent tree is enabled.
If one or more pages are invalid then the task will exit abnormally.
'ALL' performs validation when a job terminates.
'POST' performs validation only if the POST action is executed.
Validation is a slow process.