rwhod: put max cap on status update storage

Without a max cap, someone could potentially flood our memory by posting
tons and tons of spoofed status updates.

src/server/rwhod.rs

@@ -1,14 +1,14 @@
 mod packet_receiver;
 mod packet_sender;
 mod rwhod_status;
+mod status_registry;
 
 pub use packet_receiver::rwhod_packet_receiver_task;
 pub use packet_sender::{determine_relevant_interfaces, rwhod_packet_sender_task};
 pub use rwhod_status::{generate_rwhod_status_update, generate_rwhod_user_entries};
+pub use status_registry::RwhodStatusRegistry;
 
-use std::{collections::HashMap, sync::Arc};
+use std::sync::Arc;
 use tokio::sync::RwLock;
 
-use crate::proto::WhodStatusUpdate;
-
-pub type RwhodStatusStore = Arc<RwLock<HashMap<String, WhodStatusUpdate>>>;
+pub type RwhodStatusStore = Arc<RwLock<RwhodStatusRegistry>>;