RFC23, RFC24, RFC26 now use variable-length command names that
end in null octet (valid C strings) instead of fixed-length
space padded strings.

Simplify ZMQ_STREAM socket implementation

Problem: ROUTER+RAW was not a clean API

- designed for TCP clients and servers
- added HTTP client / server example in tests/test_stream.cpp
- same as ZMQ_ROUTER + ZMQ_ROUTER_RAW + ZMQ_ROUTER_MANDATORY
- includes b893ce set ZMQ_IDENTITY on outgoing connect
- deprecates ZMQ_ROUTER_RAW

Store identity for raw socket

Identity stored during connect procedure. Can be read using
zmq_getsockopt and used as the identity frame when sending messages.
This allows the implementation of a raw socket client.

Rename parse_properties to parse_metadata

Refactor how properties are parsed

Add test for CURVE mechanism

Test is skipped if no libsodium.
Added warning if libsodium not configured

Small code cleanup

Make ZAP optional for CURVE mechanism

Make ZAP optional for CURVE mechanism

Problem: allows CURVE security even if libzmq was built without libsodium

Fix test_disconnect_inproc to work on ILP64 systems

Small stream engine issue and test_monitor tidy up

Also tidy up monitor test a little.

Implement CurveZMQ message encryption and authentication 

Problem: code isn't setting mechanism to CURVE properly

Stop ZMTP handshake when an unexpected message comes