Nested wcars use the same connection.

With this change, things like this will behave reliably in multi-threaded code:
~~~.clj
(wcar r 
   (let [v    (second (wcar r (car/watch k) (car/get k)))
          v2   (do-something-with v)
          r     (wcar r (car/multi) (car/rpush k2 v2) (car/exec))]
  (println (if (nth r 2) "succeeded" "failed"))))
~~~
Without this change, it is possible that two different connections will be used, potentially allowing this ```multi``` to succeed when it shouldn't and causing another contemporaneous ```multi``` to fail when it shouldn't.