move message poller to back ground thread

SessionMessagingKit/Sending & Receiving/Pollers/Poller.swift

@@ -55,10 +55,12 @@ public final class Poller : NSObject {
             guard let strongSelf = self, strongSelf.isPolling else { return }
             Timer.scheduledTimer(withTimeInterval: Poller.retryInterval, repeats: false) { _ in
                 guard let strongSelf = self else { return }
+                DispatchQueue.global().async {
                     strongSelf.setUpPolling()
                 }
             }
         }
+    }
 
     private func pollNextSnode(seal: Resolver<Void>) {
         let userPublicKey = getUserHexEncodedPublicKey()