handle chunked parts across multiple reads

fixes https://redmine.lighttpd.net/issues/2554
where Happstack performs multiple writes to send HTTP chunked header,
HTTP chunked body, and CRLF ending HTTP chunked body

supercedes https://github.com/lighttpd/weighttp/pull/8

handle additional chunked case of split CR and LF
reference
  https://github.com/lighttpd/weighttp/pull/9/files#r52358620
  thx stbuehler

detect CR not followed by LF (error)

handle NIL in chunked stream
(should not occur in HTTP headers or chunked header,
 but could occur in chunked body)

x-ref:
  "weighttp closing connection too early(?)"
  https://redmine.lighttpd.net/issues/2554

github: closes #8, closes #9

When i is increased to (config.thread_count - 1), rest_concur has been decreased to zero.
Therefore diff is always 1 in loop. It is unnecessary to calculate diff.

insensitively. This fixes the problem where responses get incorrectly
marked as errors if the response header field name case doesn't happen
to match to what is in the code.

Also compare connection and transfer-encoding tokens case insensitively.

(Reference: RFC 2616: header field names are not case sensitive.)

Co-authored-by: Stefan Bühler <stbuehler@web.de>

fix client state machine