If request headers are received in incremental partial chunks, hyper
would restart parsing each time. This is because the HTTP/1 parser is
stateless, since the most common case is a full message and stateless
parses faster.

However, if continuing to receive more partial chunks of the request,
each subsequent full parse is slower and slower. Since partial parses is
less common, we can store a little bit of state to improve performance
in general.

Now, if a partial request is received, hyper will check for the end of
the message quickly, and if not found, simply save the length to allow
the next partial chunk to start its search from there. Only once the end
is found will a fill parse happen.

Reported-by: Datong Sun <datong.sun@konghq.com>