Async fill null values in query

Gonna punt on the full smoothing between stepped values / general response smoothing for right now - proving to be a relatively complex problem.
 - `fill:false` in options does not 'fill nulls'
 - `fill:true` in options fills null values either (a) interpolated between two points, or (b) extrapolated out from first/last non-null
 - only "fills" number returns
 - a query of all nulls will still return null