multipart_t send/recv inconsistent with socket_t

[jwmelto]

The interface to zmq::socket_t was updated to deprecate using the C flags (such as ZMQ_DONTWAIT), preferring instead enum classes (zmq::recv_flags::dontwait).

However, the zmq::multipart_t::send and recv functions still take integer flags arguments, instead of the enumerations.

It seems that, for consistency, these methods should use the enum classes.