Fix queue failure, refactor it and add a bunch of tests

[pietroalbini]

This PR fixes the documentation builder failing to get the next crate out of the queue, refactors the whole queue handling out of DocBuilder to allow easier tests, unifies all the queue handling into the new BuildQueue and puts the number of attempts in the configuration. Along with the changes, tests were added for everything.

This PR is best reviewed commit-by-commit.
r? @jyn514

[pietroalbini]

@Nemo157 addressed or responded to all review comments.

[jyn514]

Going to also run a local build to make sure things still work.

[jyn514]

Oh wait, I can't run a local build because that doesn't go through the queue 🙃 hopefully your tests exercise everything.

Can you also rebase over master so the Docker failure will go away?

[Nemo157]

docker-compose run web queue add should work I think? (Or is the queue builder also not enabled)

[jyn514]

docker-compose run web queue add should work I think? (Or is the queue builder also not enabled)

The only way to enable the queue builder is by running the daemon, which tries to build all 300,000 crates off of crates.io. Just downloading that number of crates takes several hours and it's a real waste of bandwidth.

It would be great to refactor the builder and the daemon to be separate at some point.

[pietroalbini]

Addressed the feedback and moved the code from src/queue.rs to src/build_queue.rs.