change generated sql to improve performance

[myguidingstar]

eg make use of with for subselects in N+1 queries

[myguidingstar]

replace currently generated queries like:

(SELECT ("cow"."color") AS "cow/color", ("cow"."index") AS "cow/index" FROM "cow" WHERE (("cow"."index")=(10)) LIMIT 5)
  UNION ALL
(SELECT ("cow"."color") AS "cow/color", ("cow"."index") AS "cow/index" FROM "cow" WHERE (("cow"."index")=(20)) LIMIT 5)

with:

WITH walkable_common_join AS (
SELECT ("cow"."color") AS "cow/color", ("cow"."index") AS "cow/index" FROM "cow"
)
  (SELECT * FROM walkable_common_join WHERE (("cow/index")=(10)) LIMIT 5)
  UNION ALL
  (SELECT * FROM walkable_common_join WHERE (("cow/index")=(20)) LIMIT 5)

[myguidingstar]

The use of CTEs should be optional, or even configurable for each join keyword because:

• old versions of some db don't support CTEs at all
• CTE performance characteristic of different dbs are different, eg https://blog.2ndquadrant.com/postgresql-ctes-are-optimization-fences/ (courtesy of @parameme)