Clarifying `start` and `length` in `ReadRel`

[save-buffer]

Hi, I just came a cross an issue when running Velox on a parquet file. My plan omitted the start and length fields of the LocalFiles message which caused both to be set to 0, which meant that Velox read 0 bytes at offset 0, producing no results. I think in a huge percentage of cases people will want to read the entire file. Would it be reasonable to specify these fields as optional or to make 0 for the length mean to read the whole thing? I think technically Velox is conformant right now, but this isn't the behavior I'd expect.