fix(enginev2): Close pipeline before building query results

[benclive]

What this PR does / why we need it:
Fix stats propagation by ensuring the query pipeline is closed before building the results.

• Stats are only aggregated in the pipeline close method, so this must be done before returning results but we were previously doing it in a defer, after the result object was completed.
• In order to ensure we do always close the pipeline even as this code changes over time, I still deferred it but wrapped it in OnceFunc so it doesn't get called twice. It's not really a problem to call it twice right now but I decided we should avoid this edge case in the future.

[rfratto]

Tiny improvement :) (I think this works)

Suggested change

	closePipeline := sync.OnceFunc(func() {
	pipeline.Close()
	})
	defer closePipeline()
	closePipeline := sync.OnceFunc(pipeline.Close)
	defer closePipeline()

Also, maybe we should update the Pipeline interface to note that Close must be safe to call multiple times? I think we can keep the sync.OnceFunc here, but it would be nice to solidify that as part of the contract so we don't have to worry about it every time