move open group polling to background thread

SessionMessagingKit/Sending & Receiving/Pollers/OpenGroupPollerV2.swift

@@ -23,11 +23,15 @@ public final class OpenGroupPollerV2 : NSObject {
             guard let strongSelf = self else { return }
             strongSelf.hasStarted = true
             strongSelf.timer = Timer.scheduledTimer(withTimeInterval: strongSelf.pollInterval, repeats: true) { _ in
+                DispatchQueue.global().async {
                     self?.poll().retainUntilComplete()
                 }
+            }
+            DispatchQueue.global().async {
                 strongSelf.poll().retainUntilComplete()
             }
         }
+    }
 
     @objc public func stop() {
         timer?.invalidate()