Build got stuck during running Windows tests

http://coin.intra.qt.io/coin/integration/qt/qtbase/tasks/1580442725

The integration should have been cancelled due to a timeout (heartbeat should have expired), but according to Coin logs nothing was done to dispose the VM until manual cancellation only 6 hours later.

Scheduler should automatically cancel the VM if it does not report to the scheduler process. For yet unknown reason, the agent heartbeat watchdog did not go to invalid state as it should have.