Optimize json processing and preloading

- Preload events when added to the EventHolder queue
- Remove relative time formatting from preloader. Just do it when event appears
- Process incoming json in a background queue by default

Changelog-Fixed: Fix wrong relative times on events
Changelog-Changed: Preload events when they are queued

damus/Util/EventHolder.swift

@@ -13,6 +13,7 @@ class EventHolder: ObservableObject, ScrollQueue {
     @Published var events: [NostrEvent]
     @Published var incoming: [NostrEvent]
     var should_queue: Bool
+    var on_queue: ((NostrEvent) -> Void)?
     
     func set_should_queue(_ val: Bool) {
         self.should_queue = val
@@ -35,6 +36,15 @@ class EventHolder: ObservableObject, ScrollQueue {
         self.events = []
         self.incoming = []
         self.has_event = Set()
+        self.on_queue = nil
+    }
+    
+    init(on_queue: @escaping (NostrEvent) -> ()) {
+        self.should_queue = false
+        self.events = []
+        self.incoming = []
+        self.has_event = Set()
+        self.on_queue = on_queue
     }
     
     init(events: [NostrEvent], incoming: [NostrEvent]) {
@@ -42,6 +52,7 @@ class EventHolder: ObservableObject, ScrollQueue {
         self.events = events
         self.incoming = incoming
         self.has_event = Set()
+        self.on_queue = nil
     }
     
     func filter(_ isIncluded: (NostrEvent) -> Bool) {
@@ -76,6 +87,8 @@ class EventHolder: ObservableObject, ScrollQueue {
             return false
         }
         
+        on_queue?(ev)
+        
         has_event.insert(ev.id)
         
         incoming.append(ev)