Queue links rather than checks?

[kevindew]

This isn't definitively an issue, more a design choice and probably ties into the solution for #57

Currently we queue checks, but I'd have thought it would make more sense to queue for links and then when the job starts check if there are any pending Checks for a link to decide whether to run. Can complete all the pending checks for a Link in one go.

Could also use sidekiq unique jobs to limit us to queuing once per link and running same link at same time.