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.

git-svn-id: https://develop.svn.wordpress.org/trunk@26549 602fd350-edb4-49c9-b593-d223f7449a82