WIP: CookiesMiddleware: add "reset_cookies" meta to clear the jar

[immerrr]

This PR adds a reset_cookies meta to clean the active cookiejar.

When I try working with sessions I often find myself in a situation when I'd like to restart the session "from scratch," including cookies. It's doable by just setting cookiejar meta to an arbitrary value, but then we'd be accumulating the cookiejars over time.

I'm not entirely sure about the meta name, as I'd probably like it namespaced, e.g. cookies_reset, but there's a precedent already with dont_merge_cookies, so I chose to follow that pattern.

@kmike am I missing something here?

[codecov]

Codecov Report

Merging #2986 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2986      +/-   ##
==========================================
- Coverage   84.59%   84.57%   -0.02%     
==========================================
  Files         164      164              
  Lines        9249     9251       +2     
  Branches     1376     1377       +1     
==========================================
  Hits         7824     7824              
- Misses       1167     1168       +1     
- Partials      258      259       +1

Impacted Files	Coverage Δ
scrapy/downloadermiddlewares/cookies.py	94.02% <0%> (-2.9%)	⬇️

[kmike]

I'd love to see cookiejar exposed to user directly instead of adding these "commands" in request.meta (thought on #1878 are welcome!). What's the use case? Note that with reset_cookies you're restarting from scratch at arbitrary point of time, when Request is started to download, not when you send a Request. Is it what you really want, or do you really want to clear cookiejar when sending a request?

If we're going down this request.meta route, the implementation looks fine. As for the name, what about cookiejar_reset, to use some namespace? We have many names prefixed with dont_, but other prefixes ar not common.

[immerrr]

What's the use case?

I think I've mentioned it in the OP, I'm doing sessions on top of Crawlera and want to clean up "banned" cookiejars instead of generating new ones and accumulating the old ones over time.

Request is started to download, not when you send a Request. Is it what you really want, or do you really want to clear cookiejar when sending a request?

I don't care much about it, as long as the outgoing request is clean it's fine.

[immerrr]

@kmike anything else to fix here? (aside from adding tests)