Fast comms (WIP)

[darrnshn]

This was a major refactor to make the comms system faster 🚀

Major Changes:

• Renamed worker to agent and minion to worker.
• We now use a binary protocol for all messages.
• Extracted cpp worker to its own separate repo.
• Requesters cannot specify job types. Batch jobs uses all job types.
• Single address for messages. Before, we used a vector<string> for messages, but we never actually needed more than one address.
• Heartbeats
  • Heartbeats are negotiated with a HELLO/WELCOME exchange. Before, there were 3 magic numbers for heartbeat settings per router. Now it's only 1 number: how long before a timeout. Poll rates are automatically determined from the heartbeat timeouts.
  • Heartbeats run in the same thread as the rest. Less context switches and no need for running flag.
  • Heartbeats run at the socket level, so it's somewhat transparent.
• Routers now use CRTP to statically dispatch messages to message handlers. No more router.binds.
• Added lots of tests. The interfaces are also more testable now.

Stuff left to do:

• Job types for agent.
• Add tests for heartbeating
• Add tests for timeouts
• Add tests for sad paths for delegator and agent
• Simpler job scheduling
• Add server bin
• Remove ununsed files
• Remove zmq_addon since we don't use it that much.
• Use meta::apply for router poll to avoid function lookup.
• Syntactic sugar for serialising and deserialising.