Chapter 5 espresso C code seems old

[digikar99]

I am using Ubuntu 18.04 that packs v4.1.

From http://czmq.zeromq.org/czmq4-0:czmq:

These classes are deprecated:

• zctx(3) - working with ZeroMQ contexts
• zsocket(3) - working with ZeroMQ sockets (low-level)
• zsockopt(3) - get/set ZeroMQ socket options
• zthread(3) - working with system threads
• zauth_v2(3) - authentication for ZeroMQ servers
• zbeacon_v2(3) - LAN discovery and presence
• zmonitor_v2(3) - socket event monitor
• zproxy_v2(3) - zmq_proxy wrapper

While http://zguide.zeromq.org/c:espresso does use several of these