Add unicorn model of concurrency. Refers #1.

Not sure I am doing something correctly. But, here is the example simulation code:

```clojure
(defn unicorn-worker
  "A pool of n unicorn workers."
  [n]
  (cable 2
    ; simulates intelligent routing with lb-min-conn
    (lb-min-conn
      ; a queue in front of multiple workers
      (worker-pool n
        ; a worker can only take one request at a time
        (mutex
          (delay-fixed 20
            (delay-exponential 100
              (server :rails))))))))
```