fix: change query from aggregate to find for fileblocks

- BUGFIX: mongo's create_file_block_generator had a default limit
  of 80 being set, even if one wanted to download the entire file.
  Due to this, any file download would short circuit to only 80 blocks.
  This patch fixes the issue, by assigning limit to be 0, incase limit
  is not specified

- mongomock's aggregate patch has now been removed, since there was no
  need for it after removing the aggregate query and replacing it with a
  find

- Remove logic that would add all the given file's blocks to the files
  collection on finalization, since that data was already present in the
  fileblocks collection. Instead, finalize_file would check for errors
  in the file that is to be finalized, and just set finalized to be
  'True' or 'False' for the corresponding fileid in the files
  collection.