Use subqueries for counting total entries

(cherry picked from commit 62681ff)

lib/scrivener/paginater/ecto/query.ex

@@ -28,19 +28,10 @@ defimpl Scrivener.Paginater, for: Ecto.Query do
   end
 
   defp total_entries(query, repo) do
-    primary_key =
-      query.from
-      |> elem(1)
-      |> apply(:__schema__, [:primary_key])
-      |> hd
-
     total_entries =
       query
-      |> exclude(:order_by)
-      |> exclude(:preload)
-      |> exclude(:select)
-      |> exclude(:group_by)
-      |> select([m], count(field(m, ^primary_key), :distinct))
+      |> subquery
+      |> select(count("*"))
       |> repo.one
 
     total_entries || 0