ContextManager triggers libzmq error: invalid argument, mutex.hpp: 142

[SalvoVirga]

Not a real issue, I just put it here for documentation.

The new thread-safe singleton ContextManager class (see #22), triggers an error in libzmq when that is build with Tweetnacl.

Tweetnacl is the a library for cryptography used by libzmq as default option in their CMake, but it is not recommended for production code.
A better alternative is to build libzmq with libsodium and use that.

Anyhow, since we don't use any of those, we can just build libzmq without any support for cryptography.
That means turning off the CMake option ENABLE_CURVE (i.e. -DENABLE_CURVE=OFF).

More on this at zeromq/libzmq#2991

Wiki pages about installation will change according to this and refer to this issue.