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/delegation.rs

@@ -84,7 +84,7 @@ pub struct ConditionQuery {
 }
 
 impl ConditionQuery {
-    pub fn allows_event(&self, event: &Event) -> bool {
+    #[must_use] pub fn allows_event(&self, event: &Event) -> bool {
         // check each condition, to ensure that the event complies
         // with the restriction.
         for c in &self.conditions {
@@ -101,7 +101,7 @@ impl ConditionQuery {
 }
 
 // Verify that the delegator approved the delegation; return a ConditionQuery if so.
-pub fn validate_delegation(
+#[must_use] pub fn validate_delegation(
     delegator: &str,
     delegatee: &str,
     cond_query: &str,
@@ -133,8 +133,8 @@ pub fn validate_delegation(
 }
 
 /// Parsed delegation condition
-/// see https://github.com/nostr-protocol/nips/pull/28#pullrequestreview-1084903800
-/// An example complex condition would be:  kind=1,2,3&created_at<1665265999
+/// see <https://github.com/nostr-protocol/nips/pull/28#pullrequestreview-1084903800>
+/// An example complex condition would be:  `kind=1,2,3&created_at<1665265999`
 #[derive(Serialize, Deserialize, PartialEq, Eq, Debug, Clone)]
 pub struct Condition {
     pub field: Field,
@@ -144,7 +144,7 @@ pub struct Condition {
 
 impl Condition {
     /// Check if this condition allows the given event to be delegated
-    pub fn allows_event(&self, event: &Event) -> bool {
+    #[must_use] pub fn allows_event(&self, event: &Event) -> bool {
         // determine what the right-hand side of the operator is
         let resolved_field = match &self.field {
             Field::Kind => event.kind,
@@ -323,7 +323,7 @@ mod tests {
                 Condition {
                     field: Field::CreatedAt,
                     operator: Operator::LessThan,
-                    values: vec![1665867123],
+                    values: vec![1_665_867_123],
                 },
             ],
         };