Use r.client.disconnect instead of r.quit to close connections.

We have two requirements when closing connections:

1) Don't call QUIT as it is useless added latency.
2) Don't wait for the GC finalizer to get our process socket closed.

Disconnecting the client object inside the Redis object should do the
trick.

cluster.rb

@@ -125,7 +125,7 @@ def close_existing_connection
             @connections.each{|n,r|
                 @connections.delete(n)
                 begin
-                    r.quit
+                    r.client.disconnect
                 rescue
                 end
                 break
@@ -152,7 +152,7 @@ def get_random_connection
                     @connections[n[:name]] = r
                     return r
                 else
-                    r.quit
+                    r.client.disconnect
                 end
             rescue => e
                 # Just try with the next node.