Introduce support for nested queries in WP_Meta_Query.

Previously, meta query arguments could be joined by a single AND or OR relation.
Now, these queries can be arbitrarily nested, allowing clauses to be linked
together with multiple relations.

Adds unit tests for the new nesting syntax. Modifies a few existing unit tests
that were overly specific for the old SQL syntax. Backward compatibility with
existing syntax is fully maintained.

Props boonebgorges, DrewAPicture.
See #29642.

git-svn-id: https://develop.svn.wordpress.org/trunk@29887 602fd350-edb4-49c9-b593-d223f7449a82