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

...T_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.

This should fix the compile issue.  I will be submitting a test case tomorrow (likely).