Temporarily skip queues on too many throttled jobs in a row

[ixti]

Brings back "cooldown" queues in case they return throttled jobs only functionality. One can configure the behaviour with:

Sidekiq::Throttled.configure do |config|
  # Period in seconds to exclude queue from polling in case it returned
  # {config.cooldown_threshold} amount of throttled jobs in a row. Set
  # this value to `nil` to disable cooldown manager completely.
  # Default: 2.0
  config.cooldown_period = 2.0

  # Amount of throttled jobs returned from the queue subsequently after which
  # queue will be excluded from polling.
  # Default: 1 (cooldown after first throttled job)
  config.cooldown_threshold = 1
end

Related: e5ac585
Closes: #160