def resolve_retry_delay_proc(proc)
      if proc.respond_to?(:queue_retry_delay_proc)
        resolve_retry_delay_proc(proc.queue_retry_delay_proc)
      elsif proc.is_a?(Proc)
        proc
      else # default
        Backburner.configuration.retry_delay_proc
      end
    end
Resolves retry delay proc based on the value given. Can be proc, a class or nothing

@example
resolve_retry_delay_proc(proc) => proc
resolve_retry_delay_proc(FooBar) => <queue retry_delay_proc>
resolve_retry_delay_proc(nil) => <default retry_delay_proc>