This commit's goal is to add a cop (Scalability::IdempotentWorker)
that will catch all workers that don't call idempotent! in its scope.

Calling it will label the worker as idempotent, which will end up in
our Sidekiq logs for further visibility.

This also introduces a shared example and a perform_multiple helper
method for executing jobs multiple times through unit tests.

Also in this commit we set an example in an already idempotent worker
ExpireJobCacheWorker. For this worker we call idempotent! in its
scope and add tests with the new test helpers.