Remove read_ltsv_record() from LTSVReader

To parse each record correctly, the parser should be able to stop
parsing, but read_ltsv_record() creates new parser everytime it's
called, and the parser relies on some internal informations to parse
LTSV correctly, thus using read_ltsv_record() affects the next record
and introduces program error. Let's take a different approach to support
safe incremental parsing of LTSV record.