ISPN-3312 DistributedExecutionCompletionTest.testBasicTakeInvocationWithSleepingCallable random failures

Start timing the task execution before submitting it to the executor.
ISPN-3314 DistributedTaskPart.isDone() doesn't return true unless get() was called before

A single 'done' flag is enough, the extra 'callCompleted' flag was complicating things.