enable CSV Reader to ignore invalid lines like an empty line at the end and comments - FLINK-1208

Corresponding to issue FLINK-1208:
enable CSV Reader to ignore invalid lines like an empty line at the end, multiple header lines or comments without throwing an exception
We try to avoid object instantiations as much as possible and parse the data as a `byte[]`. This effort is voided if we make the line a String for the purpose of comment checking.