Make topics bounded size

[thedevbirb]

Currently, pub/sub topics are typed as String. This leads to unnecessary, redundant data to be sent with every packet. The proposed change is to make a topic of fixed size. One approach could be to keep to implement a 4-byte selector over the hash of a topic i.e. hash(topic)[0..4].

Then, inside pubsub::Message, we would make the following internal change:

impl Message {
    /// Creates a new message with the given sequence number, topic, and payload.
    /// If the payload is empty, the server will interpret this as a subscription toggle
    /// for the given topic. The timestamp is set to the current UNIX timestamp in microseconds.
    ///
    /// # Panics
    /// Panics if the topic is larger than 65535 bytes.
    #[inline]
    pub fn new(seq: u32, topic: Bytes, payload: Bytes, compression_type: u8) -> Self {
        Self {
            header: Header {
                compression_type,
                topic_size: u16::try_from(topic.len()).expect("Topic too large, max 65535 bytes"), // <-- this would be dropped
                topic, // <---- you use the 4-byte hash here
                timestamp: unix_micros(),
                seq,
                size: payload.len() as u32, <--- maybe this won't be needed anymore since header size is always fixed?
            },
            payload,
        }
    }

[thedevbirb]

Important note is that we should somehow keep support for topic filtering.