* Add spec for precompute feed service

* Refactor PrecomputeFeedService

* spec wip

make regeneration worker unique, (only schedule/execute once at a time)

must be added to the Sidekiq invokation in your systemd file

The pull queue will handle link crawling, thread resolving, and OStatus
processing. Such tasks are more likely to hang for a longer time (due to
network requests) so it is more sensible to not make the "in-house" tasks
wait for them.