fix(lib): route on the full four-byte message code

Dispatch read one byte of the code, so a code with a non-zero high byte
was handled as its low byte. Message::get_message_code now reads the
whole little-endian uint32 and handlers register by u32. PeerInit and
PierceFirewall, the only frames with a one-byte code, get their own
accessor for the listener's first frame.

soulseek-rs-lib/src/message/server/parent_speed_ratio.rs

@@ -10,7 +10,7 @@ pub struct ParentSpeedRatioHandler;
 
 // The server sends us a speed ratio determining the number of children we can have in the distributed network. The maximum number of children is our upload speed divided by the speed ratio.
 impl MessageHandler<ServerMessage> for ParentSpeedRatioHandler {
-    fn get_code(&self) -> u8 {
+    fn get_code(&self) -> u32 {
         84
     }