Give plugin its own Redis config namespace. Since we're using defines, user can chain defines as needed.

* Port Redis PECL library connection from [Redis Object Cache](https://github.com/ericmann/Redis-Object-Cache/commit/b768b03f9dce21edd91806dc1ca741aa9523db7e)
* Replace meta functions with appropriate Redis calls
* Override default session manager

From https://core.trac.wordpress.org/browser/trunk/src/wp-includes/session.php?rev=29635