Spark tasks which cause JVM to exit with a zero exit code may cause app to hang in Standalone mode

If you have a Spark standalone cluster which runs a single application and you have a Spark task which repeatedly fails by causing the executor JVM to exit with a zero exit code then this may temporarily freeze / hang the Spark application.

For example, running

        sc.parallelize(1 to 1, 1).foreachPartition { _ => System.exit(0) }

on a cluster will cause all executors to die but those executors won't be replaced unless another Spark application or worker joins or leaves the cluster. This is caused by a bug in the standalone Master where schedule() is only called on executor exit when the exit code is non-zero, whereas I think that we should always call schedule() even on a "clean" executor shutdown since schedule() should always be safe to call.