Fixed compile issue with missing member of socket_base.  Changed ZMQ_NEXT_IDENTITY to ZMQ_NEXT_CONNECT_PEER_ID.

Fixed case where ZMQ_NEXT_CONNECT_PEER_ID is used in ROUTER, and ROUTER does not read the identity message from the connected pipe.
Merge pull request #838 from Prarrot/master

Fixed compile issue with missing member of socket_base.  Changed ZMQ_NEX...