Checking backupCursorKeepAliveFuture outside of the _mutex is racy.

One possible fix is to:

• Mark backupCursorKeepAliveFuture as (M).
• Move the check of _syncingFilesState.backupCursorKeepAliveFuture up into the _mutex.
• If the check succeed, we need to wait AFTER the _mutex to avoid risks of deadlocks.
• Add an if (_initialSyncCancellationSource.token().isCanceled()) in _keepBackupCursorAlive before setting the future.