link_checking: prevent rate-limiting

[angristan]

Fix for #1056.

• assign all links for a domain to the same thread
• reduce number of threads from 32 to 8
• add sleep between HTTP calls

Demo

devenv :: ~/dawn-zola ‹main› » time ~/zola/target/debug/zola check
Checking site...
Checking 0 internal link(s) with an anchor.
Checking 502 external link(s).
Thread for domain: soundiiz.com
Thread for domain: grafana.com
Thread for domain: web.archive.org
[...]
Thread for domain: docs.docker.com
Domain: docs.docker.com, url: "https://docs.docker.com/docker-hub/github/"
Domain: docs.docker.com, url: "https://docs.docker.com/engine/reference/commandline/login/#provide-a-password-using-stdin"
Thread for domain: wiki.diasporafoundation.org
Domain: wiki.diasporafoundation.org, url: "https://wiki.diasporafoundation.org/Installation/Debian/Jessie"
Thread for domain: core.telegram.org
Domain: core.telegram.org, url: "https://core.telegram.org/bots"
Domain: grafana.com, url: "https://grafana.com/plugins?type=datasource"
Domain: twitter.com, url: "https://twitter.com/fuolpit"
Domain: docs.docker.com, url: "https://docs.docker.com/storage/storagedriver/zfs-driver/"
Thread for domain: caniuse.com
Domain: caniuse.com, url: "https://caniuse.com/#feat=tls1-3"
Domain: twitter.com, url: "https://twitter.com/torproject?ref_src=twsrc%5Etfw"
[...]
Domain: github.com, url: "https://github.com/hyperic/sigar/issues/74"
Domain: github.com, url: "https://github.com/Angristan/dockerfiles/tree/master/diaspora"
Domain: github.com, url: "https://github.com/angristan/dockerfiles/tree/master/diaspora"
Domain: github.com, url: "https://github.com/caddyserver/caddy/issues/2080"
Domain: github.com, url: "https://github.com/caddyserver/caddy/releases/tag/v0.11.5"
Domain: github.com, url: "https://github.com/sivel/speedtest-cli"
Domain: github.com, url: "https://github.com/sivel/speedtest-cli#installation"
Domain: github.com, url: "https://github.com/sivel/speedtest-cli#usage"
Domain: github.com, url: "https://github.com/Microsoft/vscode/issues/51132"
Domain: github.com, url: "https://github.com/alexanderyakusik"
Domain: github.com, url: "https://github.com/Microsoft/vscode/issues/51132#issuecomment-424132330"
> Checked 502 external link(s): 1 error(s) found.
Failed to check the site
Error: Dead link in /root/dawn-zola/content/2018-03-diaspora-in-docker/index.md to https://gitlab.koehn.com/docker/diaspora: error sending request for url (https://gitlab.koehn.com/docker/diaspora): operation timed out
~/zola/target/debug/zola check  14.13s user 1.44s system 8% cpu 3:05.32 total

Considering there is 500 links to check, it takes about 2.7s per link. Out of these 500 links there are 150 github.com links, that's why we see all the github links at the end, since this is the last thread running. I didn't get any 429 Too Many Requests using this patch.

---

This is my first time using Rust 🦀, so I'm not familiar with the idiomatic way of doing things. I had a hard times with iterators, but it looks like I ended up with something that works! I look forward to some review so I can see what can be improved. 🙏 I added a few comments on things that I'm not sure how to handle.

[Keats]

LGTM, just need to remove the println

[Keats]

Thanks!