Add option for detecting stale connections

In order to detect stale persistent connections, a new option `detect_stale_connection` is added to `yrmcds.ini`. When `detect_stale_connection` is nonzero value, the constructor of `\yrmcds\Client` sends "noop" command through an existing persistent connection. If the response is correctly received, the client use the persistent connection. Otherwise, the client discards the connection and establish a new persistent connection.

As the side effect, the constructor discards all unreceived responses and results of all "quiet" commands.