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.

The reason why we choose SPDY/2 as default for SPDY prxy was due to
Chrome's window update bug. Now its fix is available in Chrome stable,
we make SPDY/3 as default.

We cannot use assert(stream_user_data) because server-pushed stream
does not have stream_user_data.

The on_ctrl_not_send_callback, on_ctrl_recv_parse_error_callback and
on_unknown_ctrl_recv_callback were added. The latter 2 callbacks are
purely debugging purpose.  In on_ctrl_not_send_callback, If sending
SYN_REPLY failed, issue RST_STREAM to avoid a stream hanging around.

If resultant length of a frame exceeds the maximum value (which is
2**24 - 1 for SPDY/2 and 3), SPDYLAY_ERR_FRAME_TOO_LARGE is used to
indicate this error. This error will be notified by
on_ctrl_not_send_callback.

When deciding whether to close the client connection, check
request_connection_close_ of Downstream in addition of
response_connection_close_. Also we only add "Connection: Keep-Alive"
header to the HTTP/1.0 or HTTP/0.9 clients.

github translates "Client/Server" in section header to "#clientserver"
fragment, but if it is referred by `Client/Server`_, it is translated
into "#client-server". It is best to avoid '/' in section header.