remove khmer::read_parsers::IParser::ParserState::thread_id

As it is unused and uninitialized. CID 1054789
Merge pull request #323 from ged-lab/cid-1054789

remove khmer::read_parsers::IParser::ParserState::thread_id
test_normalize_by_median_no_bigcount() now runs in temp dir

tests.test_normalize_by_median_no_bigcount() produced a temp file in
pwd, now modified to run test within temp dir
Changelog formatting, locally ignored file removed from gitignore