Switch to PicoHTTPParser or llhttp

http-parser is [not actively maintained](https://github.com/nodejs/http-parser/issues/522).

[cppcoffee/khttpd](https://github.com/cppcoffee/khttpd) illustrates the use of [PicoHTTPParser](https://github.com/h2o/picohttpparser) (MIT License), which is faster than the http-parser currently used in kHTTPd.

[llhttp](https://github.com/nodejs/llhttp) is also much faster than http-parser.