Introduce nested query support to WP_Date_Query.

This enhancement makes it possible to filter post, comment, and other queries
by date in ways that are arbitrarily complex, using mixed AND and OR relations.

Includes unit tests for the new syntax. In a few places, the existing unit
tests were slightly too strict (such as when checking the exact syntax of a SQL
string); these existing tests have been narrowed.

Props boonebgorges.
Fixes #29822.

git-svn-id: https://develop.svn.wordpress.org/trunk@29923 602fd350-edb4-49c9-b593-d223f7449a82