WP_Query: Swap parsing date into own method

[screamingdev]

The WP_Query::get_posts also parses the date query.
This is modular enough to swap it to its own method at last.
A proof of concept has been made and all tests are still green.

Some details and todos:

• see ​https://github.com/ScreamingDev/wordpress-develop-1/commit/6bb681e8caf4df8ba4811690ee19f12ad1755371
• Check if the whole logic can be shifted to WP_Date_Query::get_sql

Hint: The long run is about enabling WordPress to support multiple tables (per "post-type"). To achieve this some enhancements to the current wpdb and WP_Query class are needed. This issues covers a small bit of it by refurbishing the source.