Provide socket option for pub sockets to not silently drop message

To not silenty lose messages in publisher if hwm is reached a new socket option for pub sockets is added.
if set the pub socket should return EAGAIN if hwm is reached. The socket does not explicitly block, its on the publishers side to decide to drop or to wait.

This is helpful in scenarios where data is transmitted to several subscribers and not data should get lost (e.g. audio data or stream gzipped data).