Time of check to time of use bug in plutip-server.

[zmrocze]

In the handler we check if MVar is empty and then act based on the result. See here and there. There is a race if i.e. two handlers try to stop cluster, and witness it running. Then the second thread would deadlock on the just emptied MVar.

The assumption initially was that you're not gonna spam server with stop requests - but that's exactly what we do now in ctl to check if server is ready.