/**
   * Returns a snapshot of tasks currently scheduled for execution. Does not include the
   * currently-executing task unless it is also scheduled for future execution.
   */
  val scheduledTasks: List<Task>
    get() = taskRunner.withLock { futureTasks.toList() }

  /**
   * Schedules [task] for execution in [delayNanos]. A task may only have one future execution