Motivation:

Unfinished requests even after graceful shutdown period are forcivily
closed with `ClosedSessionException`. As `ClosedSessionException`
indicates that the connection was unexpectedly disconnected,
`ClosedSessionException` is not suitable for graceful shutdown.

In this PR, I propose to add `ShuttingDownException` to terminate
unfinished requests when a server stops.

Modifications:

- Introduce `GracefulShutdown` to customize graceful shutdown behavior.
  - Users can specify a error function to create an exception to
    unfinished terminate requests.
- Fixed `HttpServerHandler` to send error responses using the error
  function of `GracefulShutdown`
- Fixed `Server` to send error respones first and then close the
  connnections.
- Deprecation) `ServerConfig.gracefulShutdownQuietPeriod()` and
  `ServerConfig.gracefulShutdownTimeout()` have been deprecated in favor
  of `ServerConfig.gracefulShutdown()`.

Result:

You can now use `GracefulShutdown` to terminate unfinished requests when
a server stops.
```java
GracefulShutdown gracefulShutdown =
  GracefulShutdown
    .builder()
    .quietPeriod(Duration.ofSeconds(10))
    .timeout(Duration.ofSeconds(15))
    .shutdownErrorFunction((ctx, req) -> {
        return new ServerStopException();
    })
    .build();

Server
  .builder()
  .gracefulShutdown(gracefulShutdown);
```