Sidekiq documentation says 'Sidekiq is not designed to work well with dozens of queues.'. Having the amount of queues we have at the moment brings no anyway.

Closes #5571

Set the default limit of dead jobs to 5k, which should equal to about 50M memory
usage in the worst case. Sidekiq's default is 10k. Set the expiration
timeout to 6 weeks, matching our minor release schedule. Sidekiq's
default is 6 months.

Given the new federation code stores the XML payload in the job
parameters, fine-tuning here can give more significant memory savings.

This fixes the issue reported in #5947. Sidekiq will only log to stdout when this key evals to false, so this can't be forced with an empty string. This change eliminates the key altogether when running on Heroku.

closes #6500