Switch from libzmq to something memory safe

[ewust]

Libzmq (4.3.1) has been crashing with memory corruption/stack overflows in zmq_proxy. We've updated to 4.3.4 to hopefully help mitigate, but long term we should move away from a libraries written in C.

Potentially Redis? Some other pubsub? Ideally we can get rid of the dual zmq/redis dependency at the same time, and just use a single fast-but-safe pubsub.