Moving the `pluck` and `ids` methods to their own delegate line.

These two methods aren't really statistical helper methods and don't
really belong in any other group which is being delegated for querying,
so I'm moving them to their own group of methods.

I've also changed the `:to => :all` hash syntax to `to: :all`.
Merge pull request #11109 from wangjohn/moving_pluck_and_ids_to_new_delegate_line

Moving the `pluck` and `ids` methods to their own delegate line.