Use built-in close-on-destruct method on connection

[emil-nasso]

Instead of relying on shutdown functions, use the built-in set_close_on_destruct method on the connection. This method closes the connection in a safe way.

Using a shutdown function and calling close on the channel and connection is problematic when the server closes the connection (or there is a problem with the network connection. In that case, the following will happen:

• The application will shut down because the connection is lost.
• The registered shutdown function will be called.
• The shutdown method in Request will call close on the channel and the connection.
• These are already closed, causing an exception to be thrown.
• When exceptions are thrown in shutdown functions, nothing is able to handle them. This causes a fatal error instead.
• No other shutdown functions will be able to run as the execution will abruptly stop when the fatal error occurs.

Letting php-amqplib close the connection solves this problem as it does it in a safe way, without throwing exceptions.

[bschmitt]

@emil-nasso thank you very much, maybe you want to support this project? #41