This is for testing Nick.  You wanted to use the heartbeat interval set to 0 for the tests so that we didn't need a separate "disable heartbeats for testing" flag.

For testing we don't want to cut off the connection because then we're not isolating the test: the client or server might close the connection because it detects the connection having been shut off at the transport level instead of through the DDP pings.  For the tests, we need the code to not *respond* to a ping, so that we know it's actually the heartbeat timeout code that's getting exercised instead of some other code.

If you want the *user* to be able to disable heartbeats that's a separate feature: if you want heartbeat interval == 0 to mean the user disables heartbeats, then we'll need some *other* flag for testing ("`disableHeartbeatResponse`")