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/notice.rs

@@ -19,18 +19,14 @@ pub enum Notice {
 }
 
 impl EventResultStatus {
-    pub fn to_bool(&self) -> bool {
+    #[must_use] pub fn to_bool(&self) -> bool {
         match self {
-            Self::Saved => true,
-            Self::Duplicate => true,
-            Self::Invalid => false,
-            Self::Blocked => false,
-            Self::RateLimited => false,
-            Self::Error => false,
+            Self::Duplicate | Self::Saved => true,
+            Self::Invalid |Self::Blocked | Self::RateLimited | Self::Error => false,
         }
     }
 
-    pub fn prefix(&self) -> &'static str {
+    #[must_use] pub fn prefix(&self) -> &'static str {
         match self {
             Self::Saved => "saved",
             Self::Duplicate => "duplicate",
@@ -47,7 +43,7 @@ impl Notice {
     //    Notice::err_msg(format!("{}", err), id)
     //}
 
-    pub fn message(msg: String) -> Notice {
+    #[must_use] pub fn message(msg: String) -> Notice {
         Notice::Message(msg)
     }
 
@@ -56,27 +52,27 @@ impl Notice {
         Notice::EventResult(EventResult { id, msg, status })
     }
 
-    pub fn invalid(id: String, msg: &str) -> Notice {
+    #[must_use] pub fn invalid(id: String, msg: &str) -> Notice {
         Notice::prefixed(id, msg, EventResultStatus::Invalid)
     }
 
-    pub fn blocked(id: String, msg: &str) -> Notice {
+    #[must_use] pub fn blocked(id: String, msg: &str) -> Notice {
         Notice::prefixed(id, msg, EventResultStatus::Blocked)
     }
 
-    pub fn rate_limited(id: String, msg: &str) -> Notice {
+    #[must_use] pub fn rate_limited(id: String, msg: &str) -> Notice {
         Notice::prefixed(id, msg, EventResultStatus::RateLimited)
     }
 
-    pub fn duplicate(id: String) -> Notice {
+    #[must_use] pub fn duplicate(id: String) -> Notice {
         Notice::prefixed(id, "", EventResultStatus::Duplicate)
     }
 
-    pub fn error(id: String, msg: &str) -> Notice {
+    #[must_use] pub fn error(id: String, msg: &str) -> Notice {
         Notice::prefixed(id, msg, EventResultStatus::Error)
     }
 
-    pub fn saved(id: String) -> Notice {
+    #[must_use] pub fn saved(id: String) -> Notice {
         Notice::EventResult(EventResult {
             id,
             msg: "".into(),