code: function _scheduleRemoveIdle(){
      if (!this._removeIdleScheduled) {
          this._removeIdleScheduled = true;
          this._removeIdleTimer = setTimeout(_removeIdle.bind(this), this._opts.reapInterval);
      }
  }
docs: Schedule removal of idle items in the pool.
  
  Only one removal at a time can be scheduled.
  
  @private
  @this {PromisePool}
  
  @return {undefined}