Introduces bi-directional pipes

So far, there was a pair of unidirectional pipes between a socket
and a session (or an inproc peer). This resulted in complex
problems with half-closed states and tracking which inpipe
corresponds to which outpipe.

This patch doesn't add any functionality in itself, but is
essential for further work on features like subscription
forwarding.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/options.cpp

@@ -38,8 +38,6 @@ zmq::options_t::options_t () :
     reconnect_ivl_max (0),
     backlog (100),
     maxmsgsize (-1),
-    requires_in (false),
-    requires_out (false),
     immediate_connect (true)
 {
 }