Add session keepalive

Assure that a live session shows activity at least once every 25-29.9 seconds.
The interval is randomized to mitigate risk of some synchronized activity
bringing on a wave of activity all at once.  The spec indicated timeout is
'about' 60 seconds by default, but an implementation could lower it to 30
seconds within reason within the constraints of the spec.

Change-Id: I74dc78757b11571c23cb309c99eee667787ffc79