Implement flow control

This commit introduces the necessary changes necessary
for implementing flow control. None of the socket types
implements the flow control yet. The code will crash when
the flow control is enabled and the thw lwm is reached.

The following commits will add flow-control support for
individual socket types.

src/sub.cpp

@@ -67,6 +67,11 @@ void zmq::sub_t::xrevive (class reader_t *pipe_)
     fq.revive (pipe_);
 }
 
+void zmq::sub_t::xrevive (class writer_t *pipe_)
+{
+    zmq_assert (false);
+}
+
 int zmq::sub_t::xsetsockopt (int option_, const void *optval_,
     size_t optvallen_)
 {