problem: mechanisms are not supported

ZWS 2.0 will eventually have 4 modes:

* without mechanism, protocol ZWS2.0
* with null mechanism, protocol ZWS2.0/NULL
* with plain mechanism, protocol ZWS2.0/PLAIN
* with bearer mechanism, protocol ZWS2.0/BEARER

The bearer mechanism doesn't exist at the moment.

Libzmq only implement the no mechanism at the moment.

