Extract pool package. Add pool benchmark.

error.go

@@ -1,12 +1,15 @@
 package redis
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"net"
 	"strings"
 )
 
+var errClosed = errors.New("redis: client is closed")
+
 // Redis nil reply, .e.g. when key does not exist.
 var Nil = errorf("redis: nil")