Use CapnProto RPC instead of ZeroMQ

[garymm]

In #18 we're using ZeroMQ for communication and CapnProto for serialization. Since CapnProto comes with an RPC it seems simpler to just use CapnProto for communication too. I initially thought there was an advantage in using ZeroMQ in that it made it easier to add multiple agents, but this is already handled by the Minetest server<->client architecture I think. Each agent can connect to a separate minetest client, and the server handles all the multi-agent async stuff.

[garymm]

One advantage of the queue is being able to subscribe to it and observe the screenshots and actions, which I could imagine maybe useful for remote live observation. But really not sure how much better this is than having the agent record videos and then watching them afterwards.