Remove need for \Query::date_query_sql

[JJJ]

Because of the way the Date Query class currently works (it being inherited from WordPress) the resulting MySQL string needs to be stashed temporarily in a private variable, so that it can be referred to later in a different method.

This is not ideal for a few reasons:

• It does not match the way Meta & Compare queries work
• It is not obvious that it works this way
• It no longer needs to work this way, now that we've brought our own implementation of the Date Query in-house

It would be much better for it to work more like Meta & Compare, and simply return everything it needs all at once, and provide a subsequent get_sql() method to retrieve the query string part as needed.

[JJJ]

Marking as a Task for 1.1.0 as this is a blocker for other ongoing work.