feat: add until for request filters

This implements an additional filter criteria for selecting events
prior to some timestamp.

See https://github.com/fiatjaf/nostr/issues/39x

src/db.rs

@@ -229,6 +229,12 @@ fn query_from_sub(sub: &Subscription) -> String {
             let created_clause = format!("created_at > {}", f.since.unwrap());
             filter_components.push(created_clause);
         }
+        // Query for timestamp
+        if f.until.is_some() {
+            let until_clause = format!("created_at < {}", f.until.unwrap());
+            filter_components.push(until_clause);
+        }
+
         // combine all clauses, and add to filter_clauses
         if !filter_components.is_empty() {
             let mut fc = "( ".to_owned();

src/subscription.rs

@@ -29,6 +29,8 @@ pub struct ReqFilter {
     pub pubkey: Option<String>,
     /// Events published after this time
     pub since: Option<u64>,
+    /// Events published before this time
+    pub until: Option<u64>,
     /// List of author public keys
     pub authors: Option<Vec<String>>,
 }