Provide an explicit way to release a job with new :pri or :delay

[rewritten]

If for some reason (external and temporary) a job needs to be released, there could be a way (other than raising an Interrupt) to catch some exception and interpreting it as to release the job with specific options.

This should go with a way to distinguish in code whether it's executed by the worker or by passenger/whatever, as in the following example code

module AsyncMethods
  def anything(*args)
    do_something
  rescue => some_controlled_situation
    if defined(ASYNC_OBSERVER_WORKER)
      raise AsyncObserver::Release :delay => 10
    else
      async_send_opts :anything, {:delay => 10}, args
    end
  end
end