* A new `--host` option is available to restrict the listening IP address.
   The default value is `0.0.0.0` (meaning `listen all IP addresses`).
 * A new `--cache-ttl-in-seconds` option is available to set the time to live of cached responses, in seconds.
   The default value is `60` (meaning `1 minute`).
 * A new `--disable-trust-proxy` option is available to disable the feature
   even if it is activated by the static configuration file.
 * A new `--document-root` option is available to specify the document root.
   It is the path to the built-in Groonga admin page by default.
 * The `daemon` option is now ignored in the static configuration file.
   Now, you always have to specify `--daemon` option for the `droonga-http-server` command
   to start it as a daemon.
 * The `droonga-http-server-configure` command now shows prompts for all options always.
 * Responses for most commands are never cached.
   Now, only responses based on `search` or Groonga's `select` commands and the administration page are cached.
 * A new endopoint `/cache` is introduced to clear all response caches.
   To clear cached contents, send an HTTP `DELETE` request to the path.
 * Supports multiple Droonga Engine nodes as its backends.
   Now droonga-http-server can work like a load balancer.
 * The list of connecting Droonga Engine nodes is automatically updated
   based on the actual list of active members in the cluster.