Heartbeat:
- Bring back heartbeat.interval().
- Rename wp_disable_heartbeat_suspend() to wp_heartbeat_set_suspension().
- Rename the option for disabling suspension from options.suspend to options.suspension.
Fixes #25073.
Built from https://develop.svn.wordpress.org/trunk@26549


git-svn-id: http://core.svn.wordpress.org/trunk@26441 1a063a9b-81f0-0310-95a4-ce76da25c4cd