Bug: CMS tree filters doesn't count the correct number of children for deleted pages

This is a bug that combines Hierarchy, Versioned and LeftAndMain admins and CMSSiteTreeFilters.

This bug can be reproduced by having a large site tree with enough deleted pages in it so it doesn't
pre load all the children pages when initially opening an admin. Filter by either 'All pages including deleted'
or 'Deleted pages'. For CMS users it will look like deleted pages are gone.

The solution involves a couple of smaller fixes in both CMS and framework modules.

1) Ensure that 'numHistoricalChildren' are used instead of 'numChildren' when dealing with deleted pages
2) LeftAndMain::currentPage() deletes all the 'marking' cache previously built up by Hierarchy::markPartialTree()
3) Use Versioned::get_included_deleted() instead of raw DB queries against the DataObject tables when calculating parents in CMSSiteTreeFilter
Merge pull request #1056 from stojg/issue/cms-1049

BUG: Delete parent page, child pages cannot be found anymore