Clearing up SphinxQL function/variable contexts.

This is required because Sphinx 2.0.x doesn't seem to like column aliases for special variable references (the aliases don't come through as the column names in search results). Frustrating, but there you go.
Models with more than one index now get fixed facet counts.

At least, with Sphinx 2.1.x onwards - can't use the count() function with Sphinx 2.0.x.