pkg/agent: properly stop ReceiveStatuses goroutine
pkg/agent: properly stop ReceiveStatuses goroutine

When the node is being rebooted, the ReceiveStatuses goroutine was not being
properly stopped using the stop channel that was provided, which caused a panic
when the agent rebooted the machine.

fixes #93

sdemos/container-linux-update-operator,sdemos/container-linux-update-operator