title: scrapy blocked with redis
body: <p>I use a scrapy spider to act as a consumer,there is also a producer to produce urls  and put them in redis irregularly,my codes are as follows:</p>
  
  <pre><code>def start_requests(self): 
      for msg in self.redis_sub.listen():
          data = json.loads(msg['data'].decode('utf-8'))
          id = data['id']
          styleUrl = data['styleUrl']
          pageCount = data['pageCount']
          self.obi.public(json.dumps({'id': id, 'isSynchronized': 1}))
          yield SplashRequest(url=styleUrl, callback=self.specHome_parse,
                                  args={'wait': 5, 'timeout': 60, 'images': 0},
                                  meta={'pageCount': pageCount, 'id': id, 'dont_redirect': True})
  </code></pre>
  
  <p>the codes can receive urls normally,but when it blocked in the second line to wait for urls,the all scrapy paused,includding the requests yielded before. I supposed it should have continued run old requesets,what is the reason and how to sovle it? help me please</p>