Handle concurrent load of file archives properly.

Kai Reinhard

30.02.2018 0703e4eeef5bc069b744eb7a9f63449fb8524766

 borgbutler-core/src/main/java/de/micromata/borgbutler/cache/ArchiveFilelistCache.java

@@ -48,6 +48,7 @@
                    return;
                }
                savingFiles.add(file);
                Collections.sort(filesystemItems); // Sort by path.
            }
            log.info("Saving archive content as file list: " + file.getAbsolutePath());
            try (ObjectOutputStream outputStream = new ObjectOutputStream(new BufferedOutputStream(new GzipCompressorOutputStream(new FileOutputStream(file))))) {