Store an *Addr rather than a net.Addr in conn

Calling its String() method should be faster.

memcache/memcache.go

@@ -270,7 +270,7 @@ type Item struct {
 // conn is a connection to a server.
 type conn struct {
  nc net.Conn
- addr net.Addr
+ addr *Addr
  c *Client
 }