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
In order to make it more efficient with deactivated logging, we guard such statements with `if (LOG.isWarnEnabled()) { LOG.warn(...); }` Otherwise, the string concatenation will happen even with deactivated logging.