fix#171
Several names and HTTP identifiers are renamed to be
more consistent, self-explanatory, and concise:
* "Fields" is a collection of HTTP header fields (rfc7230 section 3.2)
* "Header" is the Start Line plus Fields. Another way to look at it is,
the HTTP message minus the body.
* `basic_fields` replaces `basic_headers`
* `fields` replaces `headers`
* `Fields` replaces `Headers` in template parameter lists
* `header` replaces `message_headers`
* `header::fields` replaces `message_headers::fields`
The changes are cosmetic and do not affect run-time behavior.
These changes support parsing the headers separately from the body.
* on_headers now returns void
* on_body_what is a new required callback which returns body_what
New parser set_option function for controlling independent size limits
on headers and body. By default request and response parsers are set up
with reasonable limits to prevent resource exhaustion attacks.
* Parser adheres strictly to rfc7230
* Increased test coverage
* Headers and body maximum size limit options