ZeroMQ: add czmqpp bindings as submodule

Also, add czmq bindings which are a dependency for czmqpp.
OTServerConnection: switch to czmqpp bindings

czmqpp abstracts away the ZAP (http://rfc.zeromq.org/spec:27)
handling which is necessary for the CURVE security mechanism.
MessageProcessor: switch to czmqpp bindings

czmqpp abstracts away the ZAP (http://rfc.zeromq.org/spec:27)
handling which is necessary for the CURVE security mechanism.
ZeroMQ: remove unused cppzmq submodule