ndb: switch to nostrdb notes

This is a refactor of the codebase to use a more memory-efficient
representation of notes. It should also be much faster at decoding since
we're using a custom C json parser now.

Changelog-Changed: Improved memory usage and performance when processing events

damus/Models/EventsModel.swift

@@ -41,14 +41,11 @@ class EventsModel: ObservableObject {
     }
     
     private func handle_event(relay_id: String, ev: NostrEvent) {
-        guard ev.kind == kind.rawValue else {
+        guard ev.kind == kind.rawValue,
+              ev.referenced_ids.last == target else {
             return
         }
-        
-        guard ev.referenced_ids.last?.ref_id.string() == target else {
-            return
-        }
-        
+
         if insert_uniq_sorted_event(events: &self.events, new_ev: ev, cmp: { a, b in a.created_at < b.created_at } ) {
             objectWillChange.send()
         }