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

Examples/xx-messenger/Sources/HomeFeature/HomeFeature.swift

@@ -156,6 +156,10 @@ public let homeReducer = Reducer<HomeState, HomeAction, HomeEnvironment>
             try env.messenger.logIn()
           }
 
+          if !env.messenger.isBackupRunning() {
+            try? env.messenger.resumeBackup()
+          }
+
           return .success(.messenger(.didStartRegistered))
         } catch {
           return .success(.messenger(.failure(error as NSError)))