Add size limit to server Message Codec

[the10thWiz]

This PR adds an optional size limit to the MessageCodec, which allows server to prevent clients from sending large messages that could cause the server to run out of memory.

[1tgr]

Thanks! Could you add a test? I think you can make one based on the reserves_buffer test: use the client codec to encode a message, then try to decode it with a server codec under a very small limit. The logic in the reserves_buffer test is needed since the size limit is applied when reserving extra space in the server's receive buffer: in other words, encoding then decoding the same buffer skips the check, since the buffer is already large enough to receive the incoming data.

Can you run cargo fmt? I can see that it would make some changes to the newly added lines.

[1tgr]

I think this test triggers the size limit logic correctly:

    #[test]
    fn applies_message_size_limit() {
        let message = Message::binary(
            std::iter::repeat(b"abcdefghijklmnopqrstuvwxyz")
                .take(100)
                .flat_map(|b| b.iter().copied())
                .collect::<Vec<u8>>(),
        );

        let mut data = BytesMut::new();
        MessageCodec::client()
            .encode(&message, &mut data)
            .expect("didn't expect MessageCodec::encode to return an error");

        let mut src = &data[..];
        assert_eq!(src.len(), 2608);

        let mut decoder = MessageCodec::server_with_message_size_limit(20);
        let mut decoder_buf = BytesMut::new();
        let err = loop {
            match decoder.decode(&mut decoder_buf) {
                Ok(Some(_)) => panic!("didn't expect decoder to succeed"),

                Ok(None) => {
                    let n = decoder_buf.remaining_mut().min(src.len()).min(16);
                    assert!(n > 0, "expected decoder to reserve at least one byte");
                    decoder_buf.put_slice(&src[..n]);
                    src = &src[n..];
                }

                Err(err) => break err,
            }
        };

        assert_eq!(err.to_string(), "frame exceeds size limit: 2608 bytes (a30)");
    }