Fix backend order query only to use distinct when needed

* This fixes most of the cases for redundant distinct usage. There may
  be more. But these are most common and most pressing on the DB size we
  have.

Before:

```
(325.6ms)  SELECT DISTINCT COUNT(DISTINCT "spree_orders"."id") FROM "spree_orders"  WHERE "spree_orders"."completed_at" IS NOT NULL
(0.9ms)  SELECT COUNT(DISTINCT count_column) FROM (SELECT  DISTINCT "spree_orders"."id" AS count_column FROM "spree_orders"  WHERE "spree_orders"."completed_at" IS NOT NULL LIMIT 15 OFFSET 0) subquery_for_count
  Spree::Order Load (596.3ms)  SELECT  DISTINCT "spree_orders".* FROM "spree_orders"  WHERE "spree_orders"."completed_at" IS NOT NULL  ORDER BY "spree_orders"."completed_at" DESC LIMIT 15 OFFSET 0
```

After:

```
(156.6ms)  SELECT COUNT(*) FROM "spree_orders"  WHERE "spree_orders"."completed_at" IS NOT NULL
(0.5ms)  SELECT COUNT(count_column) FROM (SELECT  1 AS count_column FROM "spree_orders"  WHERE "spree_orders"."completed_at" IS NOT NULL LIMIT 15 OFFSET 0) subquery_for_count
Spree::Order Load (0.6ms)  SELECT  "spree_orders".* FROM "spree_orders"  WHERE "spree_orders"."completed_at" IS NOT NULL  ORDER BY "spree_orders"."completed_at" DESC LIMIT 15 OFFSET 0
```

As there is no semantic change I cannot right now think about a meaningful test we could add. Asseting the generation of SQL queries comes in mind, but AR is not really prepered for this. With an in-memory adapter we might be able to do this in an ideal way.
Should this constant be frozen?