Search for a secondary pool with connections

When executing a command on a secondary node in a cluster, search the
pools for the secondaries that have available connections. This allows
the client to avoid secondary nodes that are having issues.

cluster.go

@@ -479,16 +479,25 @@ func (c *Cluster) clientForKey(key string, random, secondary bool) (Client, stri
 
 		if primAddr == "" {
 			return fmt.Errorf("could not find primary address for key %q", key)
-		} else if secondary {
+		}
+
+		if secondary {
 			for addr = range c.secondaries[primAddr] {
+				client = c.pools[addr]
+				if p, ok := client.(*pool); ok {
+					if p.conns.len() == 0 {
+						continue
+					}
+				}
+
 				break
 			}
 		}
 		if addr == "" {
 			addr = primAddr
+			client = c.pools[addr]
 		}
 
-		client = c.pools[addr]
 		return nil
 	})
 	if err != nil {