Warn when applying ordering to a query with pre-existing order

[tomekowal]

Hey!

This PR attempts to mitigate issue #522
I was thinking about removing the pre-existing order_by from queries, but Ecto docs mention that reading the internal structure of the query is somewhat OK, but modifying is a no go.

https://hexdocs.pm/ecto/Ecto.Query.html#__struct__/0

Users of Ecto must consider this struct as opaque and not access its field directly. Authors of adapters may read its contents, but never modify them.

Flop is not exactly an adapter, but I think similar logic applies.

Using the internal API of Ecto seems to me as a good trade for avoiding the confusion of accidentally passing a query with pre-existing order_by which leads to a very weird behaviour and a lot of confusion.

We could potentially be more strict and even raise when we have a query with pre-existing order_by, but that would be a breaking change.

Let me know what you think, I am happy to make any corrections you find necessary.