sendmsg and recvmsg

[gwangyi]

To implement async rep socket, in my knowledge, there are two ways.

• We can make multiple rep workers at same time. It can serve a request which is sent before replying preceding requests. However, if a req socket is connected to multiple rep sockets, request-receiving rep socket is chosen by round-robin algorithm. If one of rep workers is stuck forever, any one of requests cannot be served eventually.
• We can use raw rep socket. To correctly reply to a request, we need to use nn_sendmsg with the control header which is taken from nn_recvmsg. Raw rep sockets can recv one or more requests consequently and serve completely out of order.

For that reason, I implemented sendmsg and recvmsg in nnpy.Socket.