Problem: extended containers are non-trivial to learn

Solution: un-deprecate zlist and zhash and offer these as the
"simple" containers, with zlistx and zhashx as upgrades for
more demanding applications.
Merge pull request #812 from hintjens/master

Problem: extended containers are non-trivial to learn
Merge pull request #813 from jemc/master

Problem: CI tests many builds with one matrix itemThis
Problem: zsock and zgossip_msg don't build on ZMQv2

Solution: use a macro to emulate zmq_msg_send() and zmq_msg_recv()
and do not use zmq_send in version-independent code.
Merge pull request #815 from hintjens/master

Problem: zsock and zgossip_msg don't build on ZMQv2