Since proxy supposed to be used in different configurations
second ROUTER socket may stay unused (e.g. when we use proxy
for publishing only fanout messages in mixed direct/pub-sub
configuration see ZmqClientMixDirectPubSub).
This patch introduces two modes of proxy SingleRouterProxy (where
frontend and backend ROUTERS are the same socket) and
DoubleRouterProxy (different sockets for the frontend and for the
backend - the original behavior).
Change-Id: Ia859b92e1f238fcbbcf42e17b06e0f4ad04e79f6