improvement: add NostrRepo trait, with sqlite implementation

This is inspired by the work of
v0l (https://github.com/v0l/nostr-rs-relay/).

A new trait abstracts the storage layer with an async API.  Rusqlite
is still used with worker threads, but this allows for Postgresql or
other backends to be used.

There may be bugs, this has not been rigorously tested.

src/conn.rs

@@ -14,7 +14,7 @@ const MAX_SUBSCRIPTION_ID_LEN: usize = 256;
 /// State for a client connection
 pub struct ClientConn {
     /// Client IP (either from socket, or configured proxy header
-    client_ip: String,
+    client_ip_addr: String,
     /// Unique client identifier generated at connection time
     client_id: Uuid,
     /// The current set of active client subscriptions
@@ -32,22 +32,22 @@ impl Default for ClientConn {
 impl ClientConn {
     /// Create a new, empty connection state.
     #[must_use]
-    pub fn new(client_ip: String) -> Self {
+    pub fn new(client_ip_addr: String) -> Self {
         let client_id = Uuid::new_v4();
         ClientConn {
-            client_ip,
+            client_ip_addr,
             client_id,
             subscriptions: HashMap::new(),
             max_subs: 32,
         }
     }
 
-    pub fn subscriptions(&self) -> &HashMap<String, Subscription> {
+    #[must_use] pub fn subscriptions(&self) -> &HashMap<String, Subscription> {
         &self.subscriptions
     }
 
     /// Check if the given subscription already exists
-    pub fn has_subscription(&self, sub: &Subscription) -> bool {
+    #[must_use] pub fn has_subscription(&self, sub: &Subscription) -> bool {
         self.subscriptions.values().any(|x| x == sub)
     }
 
@@ -60,7 +60,7 @@ impl ClientConn {
 
     #[must_use]
     pub fn ip(&self) -> &str {
-        &self.client_ip
+        &self.client_ip_addr
     }
 
     /// Add a new subscription for this connection.