querystring: improve parse() performance

[mscdex]

These changes improve parse() performance from ~12-32% on all of the existing querystring benchmarks:

querystring/querystring-parse.js type=noencode n=1000000: ./node: 444400 ./node-prev: 369170 ... 20.38%
querystring/querystring-parse.js type=encodemany n=1000000: ./node: 383030 ./node-prev: 288910 . 32.58%
querystring/querystring-parse.js type=encodelast n=1000000: ./node: 392020 ./node-prev: 336360 . 16.55%
querystring/querystring-parse.js type=multivalue n=1000000: ./node: 269660 ./node-prev: 240230 . 12.25%