Add connection pools support

Add connection pools support. 

Usage:
<pre><code>;; initial connection pool
(redis:init-connection-pool :port 6347 :count 15)

;; use connection pool
(redis:with-connection-in-pool
    ;; your code
)
</code></pre>
If `(>= (length (the-pool)) *max-pool-size*)` we do nothing. Seems like the connection is leaked here