add two macros and improve performance when sending Strings

* add macro @v2only and @v3only to specify code / code blocks for different zmq version. This makes it look slightly cleaner.
* use function `zmq_send()` when sending Julia Strings, thus avoid convert String to ZMQMessage and the memcpy process. (@v3only)

(NOTE, I have only zmq3 installed right now. So the changes have not been tested with zmq2)