Close connections that are being discarded

If a TCP connection to memcache is not being returned to the free list (because an error
has occurred on the connection and its reuse is not allowed), explicitly close the
connection.

memcache/memcache.go

@@ -183,6 +183,8 @@ func (cn *conn) extendDeadline() {
 func (cn *conn) condRelease(err *error) {
 	if *err == nil || resumableError(*err) {
 		cn.release()
+	} else {
+		cn.nc.Close()
 	}
 }