Don't assume 'io.BytesIO' instances have a 'len'
Don't assume 'io.BytesIO' instances have a 'len'

I can't find in the documentation where `io.StringIO` instances have a
`len` attribute, but `io.BytesIO` instances evidently do not. Stop
trying to log the length of the chunk using `chunk.len`.

betable/tablesnap,betable/tablesnap