Backup improvements & example

XXMessengerClient library

• Fix race condition issues when starting the backup and setting initial parameters.
• Add BackupStorage utility that can store the backup.
• Store BackupStorage.Backup on disk.

xx-messenger example

• Start backup
• Resume backup
• Stop backup
• Store backups in storage
• Observe backup storage
• Export backup from storage
• Resume backup on launch

Sources/XXMessengerClient/Messenger/Functions/MessengerCreate.swift

@@ -16,7 +16,7 @@ extension MessengerCreate {
       let password = env.generateSecret()
       try env.passwordStorage.save(password)
       let storageDir = env.storageDir
-      try env.fileManager.removeDirectory(storageDir)
+      try env.fileManager.removeItem(storageDir)
       try env.fileManager.createDirectory(storageDir)
       try env.newCMix(
         ndfJSON: String(data: ndfData, encoding: .utf8)!,