In Special:AllPages, limit the size of hierarchical lists

Traversal of the entire page list by spidering the top levels is thought
to require O(N^3) DB CPU time where N is the number of pages on the
wiki. So introduce a limit for the size of such lists, as measured by
the pre-existing estimateRowCount().

Bug: 56840
Change-Id: I189ba71de869496a36f49283ec6dce7bbdfccd73