search: refactor fast_count to return nil on timeout.

* Refactor fast_count to return nil instead of 1,000,000 if the exact count times out.
* Remove the estimate_post_counts and blank_tag_search_fast_count global config options.
* Replace the hardcoded post count estimates inside fast_count with a
  method that parses Postgres's estimated row count from EXPLAIN.

* /counts/posts.json:
** Remove the `raise_on_timeout` parameter.
** Add an `estimate_count=<true|false>` parameter.
** Return null instead of 1,000,000 if the exact count times out.

app/logical/post_sets/post.rb

@@ -84,7 +84,7 @@ module PostSets
     def get_post_count
       if %w(json atom xml).include?(format.downcase)
         # no need to get counts for formats that don't use a paginator
-        return Danbooru.config.blank_tag_search_fast_count
+        nil
       else
         query.fast_count
       end
@@ -103,15 +103,11 @@ module PostSets
         if is_random?
           temp = get_random_posts
         else
-          temp = query.build.paginate(page, count: post_count, limit: per_page)
+          temp = query.build.paginate(page, count: post_count, search_count: !post_count.nil?, limit: per_page)
         end
       end
     end
 
-    def unknown_post_count?
-      post_count == Danbooru.config.blank_tag_search_fast_count
-    end
-
     def hide_from_crawler?
       return true if current_page > 1
       return false if query.is_empty_search? || query.is_simple_tag? || query.is_metatag?(:order, :rank)