scrapy wouldn't create new request after i pause crawl and resume crawl, and redis wouldn't create `spidername:requests` set again, even i set `SCHEDULER_PERSIST = True` at settings.py.

[Vickey-Wu]

pause crawl (ctrl+c) and resume crawl util 0 in redis:requests set, and when spider closed spidername:requests were deleted

127.0.0.1:6379> KEYS *
1) "spidername:requests"
2) "spidername:dupefilter"
127.0.0.1:6379> ZCARD spidername:requests
(integer) 7
127.0.0.1:6379> ZCARD spidername:requests
(integer) 0
127.0.0.1:6379> KEYS *
1) "spidername:dupefilter"
127.0.0.1:6379> SCARD spidername:dupefilter
(integer) 26

i set SCHEDULER_PERSIST = True at settings.py and try to run spider again, redis wouldn't create "spidername:dupefilter" redis set again, only dupefilter is still here.

127.0.0.1:6379> KEYS *
1) "spidername:dupefilter"
127.0.0.1:6379> SCARD spidername:dupefilter
(integer) 26

2019-07-24 06:58:05 [scrapy.core.engine] INFO: Spider opened
2019-07-24 06:58:05 [scrapy.extensions.logstats] INFO: Crawled 0 pages (at 0 pages/min), scraped 0 items (at 0 items/min)
2019-07-24 06:58:05 [spidername] INFO: Spider opened: spidername
2019-07-24 06:58:05 [scrapy.extensions.telnet] INFO: Telnet console listening on 127.0.0.1:6023
2019-07-24 06:58:16 [root] INFO: ***************** crawling page 2 ***************** 
2019-07-24 06:58:16 [scrapy.core.engine] INFO: Closing spider (finished)
2019-07-24 06:58:16 [scrapy.statscollectors] INFO: Dumping Scrapy stats:
{'downloader/request_bytes': 926,
 'downloader/request_count': 4,
 'downloader/request_method_count/GET': 4,
 'downloader/response_bytes': 8868,
 'downloader/response_count': 4,
 'downloader/response_status_count/200': 2,
 'downloader/response_status_count/301': 2,
 'finish_reason': 'finished',
 'finish_time': datetime.datetime(2019, 7, 24, 6, 58, 16, 951389),
 'log_count/INFO': 11,
 'memusage/max': 53563392,
 'memusage/startup': 53563392,
 'request_depth_max': 1,
 'response_received_count': 2,
 'robotstxt/request_count': 1,
 'robotstxt/response_count': 1,
 'robotstxt/response_status_count/200': 1,
 'scheduler/dequeued/redis': 2,
 'scheduler/enqueued/redis': 2,
 'start_time': datetime.datetime(2019, 7, 24, 6, 58, 5, 486811)}
2019-07-24 06:58:16 [scrapy.core.engine] INFO: Spider closed (finished)