Separate control and message traffic

Some API commands like START and UUID and NAME expect a reply. If
there is cluster traffic (ENTER, SHOUT, etc.) arriving at the same
time then the API gets confused.

Solution: create a second socket pair (inbox/outbox) and pass the
outbox to the zyre_node_actor. All API commands still use the pipe
as normal, and then cluster traffic comes over the outbox/inbox
pair.

Fixes #191
Merge pull request #192 from hintjens/master

Separate control and message traffic