Make IndexPager query direction code more readable

Make IndexPager query direction code more readable

Rename $descending variable in doQuery() to $order, along with the
$descending argument to reallyDoQuery() and buildQueryInfo(). Use
new IndexPager::QUERY_* constants for checking and inverting such
values. Fix the comments so that they do not imply the opposite of
what is true anymore.

For compatibility, the constants are boolean values such that any
subclass defining reallyDoQuery()/buildQueryInfo() can use that
argument the same way as before.

Change-Id: I912d3678c755c25463a2fadbec6888f3a87d4215