Check for existing rebuilds

[chrismytton]

This checks for existing jobs before re-queueing them, which should help keep the size of the rebuild queue down.

Fixes #37

Note to merger

Make sure the following have been done before merging this:

• Change the base of this pull request to master once Add Rubocop #53 has been merged.

[tmtmtmtm]

There's really two distinct types things going on here, so I think it would be worth splitting the PR in two — the first for the refactoring (which looks like it could go through like this OK) and then the second for changing the behaviour (which I think also really needs a test to show the new behaviour is working).

[chrismytton]

There's really two distinct types things going on here, so I think it would be worth splitting the PR in two — the first for the refactoring (which looks like it could go through like this OK) and then the second for changing the behaviour (which I think also really needs a test to show the new behaviour is working).

Yep, good shout. I've pulled out the refactoring into a separate PR - #55.

[tmtmtmtm]

I'm a little concerned that if I remove the if clause in the new add method — i.e. making it simply

    def add(country, legislature, source = nil)
      RebuilderJob.perform_async(country, legislature, source)
      "Queued rebuild for country=#{country} legislature=#{legislature} source=#{source}\n"
    end

then the tests still pass…

This definitely seems to be the right abstraction, but I think finding it at the end of this PR makes it a little harder to follow. I'm wondering if it might make more sense to do this as two steps — firstly a 'clean' refactoring that maintains the old behaviour, but with the new Queue implementation. Then once we know that's working correctly, adding the 'skip if one exists' logic.