Garbage collection for transients stored in the options table

[johnbillion]

When there's no persistent object cache in place, WordPress uses the options table to store transients. We should introduce garbage collection to avoid stale data sitting around indefinitely.

This should be fairly straight forward to implement. A daily cron schedule would fetch all options with a key that begin with _transient_timeout_ and a value that's less than the current time, then clear all the matching transients.

Any objections? I'll patch it up if not.

[johnbillion]

Little bit of background discussion on Mark Jaquith's blog: ​http://markjaquith.wordpress.com/2013/04/26/fragment-caching-in-wordpress/

[SergeyBiryukov]

Duplicate of #20316.