Support building nghttpx with systemd using cmake

Run nghttpx integration test with cmake build.  Autotools build erases
artifacts for packaging tests, so we cannot run nghttpx integration
test with autotools build at the moment.

Relay on go modules to download dependencies.

nghttpx: Fix unchanged log level on configuration reload

Previously, if log-level is not mentioned in configuration file and
reload happens, the log level was not set to the default value NOTICE.
Instead, the log level stayed the same.  This commit fixes this bug.

Travis osx

Ignore Content-Length and Transfer-Encoding in 200 response to CONNECT request

A well known server sends content-length: 0 in 101 response.  RFC 7230
says Content-Length or Transfer-Encoding in 200 response to CONNECT
request: https://tools.ietf.org/html/rfc7230#section-3.3.3

It returns NULL if return value is HPE_OK.

Replace http-parser with llhttp

llhttp does not include URL parser.  We extracted URL parser code from
http-parser and put it under third-party/url-parser.

llhttp bd3d224eb8cdc92c6fc8f508d7bbe0ba266e8e92

Upgrade mruby

asio: support boost-1.70

mruby minirake does not like CXX="clang++-7 -std=c++14" because it
expects CXX to be an executable.  To workaround this, split CXX so
that CXX becomes a single word and move the rest of words to CXXFLAGS.

In boost 1.70, deprecated get_io_context() has finally been removed.
Introduce GET_IO_SERVICE macro that based on boost version uses
old get_io_service() interface (boost < 1.70), or get_executor().context()
for boost 1.70+.

Commit based idea seen in monero-project/monero@17769db9462e5201befcb05f86ccbaeabf35caf8