Support basic custom queries w/automatic batching

[stephenh]

I.e. something like:

const row = em.customBatchedQuery(`
  SELECT
    ps.id,
    COUNT(*) as total_pi, 
    SUM(CASE pi.bought_out_status_id WHEN 1 THEN 1 ELSE 0 END) as bought_out_pi
  FROM project_stages ps
  LEFT OUTER JOIN project_items pi ON pi.project_stage_id = ps.id 
  WHERE ps.id IN (?)
  GROUP BY ps.id
`, ps.untaggedIdOrFail);

If invoked with N ps.untaggedIdOrFails in the same event loop, this would be 1 DB query.

The convention would be that whatever the input parameter is (the single number/text of the 2nd param), must be the 1st column of the custom query, so that the implementation would knows how to partition out the single big result back to each individual caller.

This is stock dataloader functionality, the idea is to just provide a nicer API that is also scoped to the EntityManagers existing connection/lifecycle/etc.

[stephenh]

We have em.getLoader which I think is good enough for now.