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.

test/functional/counts_controller_test.rb

@@ -7,12 +7,6 @@ class CountsControllerTest < ActionDispatch::IntegrationTest
         get posts_counts_path
         assert_response :success
       end
-
-      should "render an error during a timeout" do
-        PostQueryBuilder.any_instance.stubs(:fast_count).raises(Post::TimeoutError.new)
-        get posts_counts_path
-        assert_response :error
-      end
     end
   end
 end