Holds tarpitted requests before denying them.

Delaying a response costs the BEAM very little (the request process sleeps) and costs a scraper a connection for the whole delay. It still holds a connection on your side, so each instance holds at most :max_concurrent requests at once, counted with :atomics; beyond that, requests are denied immediately. Delays are capped at :max_delay milliseconds. Both are options of the :tarpit configuration, see Limen.Config.