Added macros which log messages from the following components are
prefixed with their component name + object pointer address:

ListenHandler: LISTEN
ThreadEventReceiver: THREAD_RECV
Upstream: UPSTREAM
Downstream: DOWNSTREAM
DownstreamConnection: DCONN
SpdySession: DSPDY

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.

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.