nghttpx: Add worker-frontend-connections option
h2load: Fix failed stream count is added multiple times

The number of failed stream is counted multiple times if several
connection attempts are made.
Update doc
doc: Add h2load-howto.rst
nghttpx: Use std::vector instead of raw array
nghttpx: Use std::unique_ptr for ListenHandler::http2session_
nghttp: Adjust header size for CONTINUATION test
nghttpx: Don't fail backend connection if one of backend request fails
nghttpx: Return 413 if request header is too large

For now, if request has request body, we'll issue RST_STREAM to inform
the peer to stop sending body.  RST_STREAM may be sent before error
page header or data, so peer may receive RST_STREAM only.
src: Update status code according to RFC 7231
Update doc
Treat larger last stream ID in GOAWAY than previous value as PROTOCOL_ERROR
Support one header field size (name + value) up to 64KiB by default

This commits changes the upper bound of one header field size (the sum
of the length of name and value) to 64KiB by default.  We may add an
option to change this upper bound in the future.
nghttpd: Use cipher suites recommended by Mozilla
nghttpd: Disable SSL_CTX_set_ecdh_auto for OpenSSL >= 1.0.2
nghttpd: Add --dh-param-file option to support DHE ciphers
Update README.rst
Add Rewriting location header field section to nghttpx how-to
Add const qualifier to nva parameter in nghttp2_hd_deflate_hd()
doc: Add HPACK API tutorial