Limit the number of pages that can be deleted in a single request.

This is needed because the Page table needs to be locked while pages are deleted for avoiding
mptt inconsitences. Deleting a large page hierarchy might cause other requests
to timeout waiting to acquire the table lock.
The table locking is done using the mutually_exclusive_on_post decorator which does a select_for_update for locking the pages table
Merge branch 'page_tree_corruptions' into support/2.3.x