Allow multiple aggregated columns per query

[dgeelen-uipath]

Note: this follows #499 (so some duplicate commits show up), but it doesn't seem possible to set the base branch correctly.

This enables the writing of queries like SELECT MIN(COLUMN), MAX(COLUMN), SUM(COLUMN) FROM TABLE. The existing behaviour for handling multiple COUNT() columns is preserved as a special case.

Following from #499, this renames the AsCount() to SelectCount(), which as pointed out in does something different. If AsCount() should be kept as-is, what would be a good way (name) to make the distinction between what AsCount() does vs. selecting aggregates of columns?