Merge branch 'stdin'
Changes stdin channel from REQ-REQ to ROUTER-DEALER, fixing
the round-robin load-balancing of stdin_requests across frontends.
stdin_requests now go to the client that made the execute_request
that prompted the stdin request.
stdin_requests from frontends that do not support stdin will raise an error,
rather than hanging on input that will never arrive.
reviewed by @fperez
closes
#673
