Heartbeat: clean up code style, better naming, better code structure. Props carldanley, props evansolomon.
Changes:
- Add connectNow() public method to trigger a connection immediately.
- Remove the "skipping" when no data to send.
- Change the default interval to 60 sec.
- Fix resetting the next connection time when changing the interval.
See #25073.

git-svn-id: https://develop.svn.wordpress.org/trunk@26169 602fd350-edb4-49c9-b593-d223f7449a82