Refactor channel closure handling

[SanHalacogluImproving]

Replaced future cancellations (for noncomplete futures) with exceptional completion using a ClosingException upon channel closure, aligning with Python and Node clients.
Added a mechanism to return a future with a ClosingException for command submissions on already closed clients. Introduced checks to see if the client has been already closed to catch it before submitting new commands.

[Yury-Fridlyand]

I'm not sure that it is an immediate transition. channel.close is an async operation so you (or user) still may be able to write something to a channel is being closed

[acarbonetto]

And if a user writes something to the channel before the channel.close() call finishes, we could get in a state where the channel is still open (and will accept a request and add a request to the list - which will never complete).
@SanHalacogluImproving let's change this a private boolean field that saves the state of the channelHandlers.

   private boolean isClosed = false;

    public boolean isClosed() {
        return this.isClosed;
    }

    public ChannelFuture close() {
        this.isClosed = true;
        callbackDispatcher.shutdownGracefully();
        return channel.close();
    }

[Yury-Fridlyand]

Try to add IT, for example

future = client.cmd()
client.close()
future.get()

[acarbonetto]

interesting spotless formatting here.
Any way to make this cleaner? Like:

        String msg =
                "Operation terminated: The closing process has been initiated for the" + " resource.";
        responses.values().forEach(future -> future.completeExceptionally(new ClosingException(msg)));
        responses.clear();

[acarbonetto]

And if a user writes something to the channel before the channel.close() call finishes, we could get in a state where the channel is still open (and will accept a request and add a request to the list - which will never complete).
@SanHalacogluImproving let's change this a private boolean field that saves the state of the channelHandlers.

   private boolean isClosed = false;

    public boolean isClosed() {
        return this.isClosed;
    }

    public ChannelFuture close() {
        this.isClosed = true;
        callbackDispatcher.shutdownGracefully();
        return channel.close();
    }

[Yury-Fridlyand]

Use AtomicBoolean which is thread safe

[acarbonetto]

Note: except that the variable is bi-directional. It doesn't matter if multiple threads set isClosed to true, it will be true.

[Yury-Fridlyand]

CI is red