Use 580 seconds as default heartbeat interval

240 seconds gives a theoretiacl 2.5 decrease in server load
(over the previous default of 600 seconds), while
580 gives a ~3% increase.

In a test that involved watching CPU usage 1000 idle connections
I observe effectively no difference between 600 and 580 values.

580 seconds is enough to cause network activity every 290 (< 300) seconds
and hopefully make F5 gear with default timeout settings happy.