problem: zeromq doesn't has a thread-safe peer to peer socket

Solution: a new socket type, called PEER. Very similar to SERVER, but can only connect to other PEERs. Also a new zmq_connect_peer method, that connect and return a routing-id in thread-safe and atomic operation

src/session_base.cpp

@@ -87,6 +87,7 @@ zmq::session_base_t *zmq::session_base_t::create (class io_thread_t *io_thread_,
         case ZMQ_GATHER:
         case ZMQ_SCATTER:
         case ZMQ_DGRAM:
+        case ZMQ_PEER:
             s = new (std::nothrow)
               session_base_t (io_thread_, active_, socket_, options_, addr_);
             break;