Title::getContentModel(): load from DB if necessary
Also don't cast $model to int in LinkCache::addGoodLinkObj(); content
model IDs are non-numeric strings, not integers, so that field was
always populated with the value 0. Because 0 is a falsy value, this
caused subsequent calls to Title::getContentModel() to return the
default model rather than the correct one.
Also (hopefully) fixed every single query that could cause a
LinkCache entry to be added without the content model.