Fix async database transaction handling

SignalServiceKit/src/Loki/Database/Storage.swift

@@ -53,9 +53,9 @@ public final class Storage : NSObject {
     public static func write(with block: @escaping (YapDatabaseReadWriteTransaction) -> Void, completion: @escaping () -> Void) -> Promise<Void> {
         let (promise, seal) = Promise<Void>.pending()
         serialQueue.async {
-            owsStorage.dbReadWriteConnection.readWrite(block)
+            owsStorage.dbReadWriteConnection.readWrite { transaction in
-            DispatchQueue.main.async {
+                transaction.addCompletionQueue(DispatchQueue.main, completionBlock: completion)
-                completion()
+                block(transaction)
             }
             seal.fulfill(())
         }