filesystem blobstore does not atomically replace objects

Object stores have atomic replacement of keys, i.e., a mutating operation like write or overwrite should succeed and expose the new object or fail and retain the old object. The filesystem blobstore does neither of these and also has issues when handling simultaneous writes. From FilesystemStorageStrategyImpl.putBlob:

try {
   outputFile.delete();
   Files.asByteSink(outputFile).writeFrom(his);
   ...
} catch (IOException ex) {
   if (outputFile != null) {
      if (!outputFile.delete()) {
         logger.debug("Could not delete %s", outputFile);
      }
   }
   throw ex;
}

Instead we should write to a temporary file and rename on top of the target object.