The logic for count aggregations for grouped data in #145 was an improvement over what was already in place for ungrouped data. Instead of leaving two separate code paths with different logic I unified logic for both to use the new logic from #145.

This allowed removing unnecessary uses of the $facet stage which forks the aggregation pipeline. Previously every aggregate used a separate facet. Now we only need facets for incompatibly-grouped data - one query that combines ungrouped aggregates with groups, or that combines either of those with field selection. This required additional changes to response processing to remove facet unpacking logic. The new system does mean that there are a couple of places where we have to explicitly fill in null or zero results for aggregate queries with no matching rows.

While I was going over aggregate logic I noticed some unescaped field references when referencing aggregate result names. I fixed these to use ColumnRef which escapes names. While I was at it I removed the last couple of uses of the old getField helper, and replaced them with the new-and-improved ColumnRef.