Simplify heartbeat API.
 * Move to a method to check connection errors, a better framework for future changes.
 * Remove start(), stop(), autostart.

props carldanley.
fixes #23216.



git-svn-id: http://core.svn.wordpress.org/trunk@24749 1a063a9b-81f0-0310-95a4-ce76da25c4cd