Message bus replacement service

The server and client currently communicate via both HTTP/rest and an external message bus. This adds a fair amount of complexity, especially around the submission of tasks where two requests are required separated by subscription to the message bus and the task worker has to keep track of multiple submitted but unstarted tasks.

A simpler approach could be to use a long lived HTTP connection that streams the events directly to the client. This could potentially be gRPC or websockets (or something else?).

We would then be left with a single `run_plan(name, args)` endpoint and could remove `submit_task`, `delete_submitted_task`, `get_tasks`, `set_active_task` and `get_task`.

This could also enable us to support aborting plans if the client exits (eg `Ctrl-C` from the terminal aborts the plan) or pausing and resuming a plan within the same process.