Generated node count limit

Generated node count limit

To prevent large template DOM caches from sending servers into swap,
throw an exception when more than some number of DOM elements are
parsed. Unfortunately, it wasn't possible to return a normal error
message, because it broke PST and extractSections and corrupted the
article text. It's safer to refuse to save the edit, and we don't
have decent ways to do that short of throwing an exception.

Ideally we would like to have an upstream patch that hooks libxml to
allocate memory from PHP's request pool, then a fatal error would be
raised instead of swapping.

Change-Id: I4cb4f6fd313e1e0940b56cc5e586afd1bea9267a