Data Executor Refactor

[ndubois-legion]

Summary

Make data-executor more scalable

Current State

Currently the data-executor executes a single job at once, even though it supports many workers. We also need to go through gRPC or OpenAPI. The RemoteStub client & the data graph must also by async, for all this to work.

Tasks

• [ ] Investigate async data graph processing #1619
• [x] First design
• [ ] #1857 ...