Fix start_requests lazyness and early hangs

- Removes new public methods added by #330 to Crawler and CrawlerProcess
- Add test for start_requests lazy evaluation
- Fix and test hangs when start_requests erroed before returning the generator
- Add test when start_requests fails while generating requests
- Simplify Crawler and CrawlerProcess implementation taking in count
  that only one spider can be attached per Crawler. As required by SEP-019
- "scrapy settings" command do not require starting a Crawler anymore