Use --no-tls option to disable SSL/TLS and specify SPDY protocol
version using -2 or -3.

The -k, --insecure option is added to skip this verification.  The
system wide trusted CA certificates will be loaded at startup. The
--cacert option is added to specify the trusted CA certificate file.

Possibly because of deferred callback, we may get this callback when
the output buffer is not empty.

With --client-proxy option, shrpx makes sure that the request path is
an absolute URI, otherwise it will return 400 status code.

In client mode, now SPDY connection to the backend server is
established per thread.  The frontend connections which belong to the
same thread share the SPDY connection.

Don't call HttpsUpstream::resume_read() from the call tree of
on_read().  Avoid parsing next http data after parse error.

With --client-mode option, shrpx now accepts unencrypted HTTP
connections and communicates with backend server in SPDY.  In short,
this is the "reversed" operation mode against normal mode.  This may
be useful for testing purpose because it can sit between HTTP client
and shrpx "normal" mode.

We should only call daemon() after ListenHandler is
instantiated, where  SSL_CTX_use_PrivateKey_file is called,
otherwise we have no stdin/stdout to get the password for
keyfile.

We should probably make this spec file a template and integrate
it with our build process (make package?). Will follow-up with
that eventually.

This change implements spdylay_map functions in iterative approach
instead of original recursive approach. The iterative version is
overall faster than recursive one. The traversal function
(spdylay_map_each) is slightly slower but it is not noticeable.