* Updated for the latest Droonga Engine 1.1.0.
 * Supports uber-cache 2.0.0.
 * New endpoints are introduced to inspect connections to Droonga Engine nodes.
   - `/engines` returns the list of currently conencted Droonga Engine nodes.
   - `/connections` returns detailed status of internal connections to Droonga Engine nodes.
 * Always reconnects to Droonga Engine nodes when there is any change around the Droonga cluster.
 * Query parameters for `/droonga/*` endpoints are used as the message body.