[manager/dispatcher] Replace call to isRunning() to isRunningLocked() in dispatcher Heartbeat()
[manager/dispatcher] Replace call to isRunning() to isRunningLocked() in dispatcher Heartbeat()

Signed-off-by: Anshul Pundir <b83733ec9a5dc60cc9eb3ca9cb86ec17acc0a878@docker.com>

dperny/swarmkit-1,docker/swarmkit,docker/swarmkit,dperny/swarmkit-1