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>