framework for durable task execution

[jgraettinger]

We have a lot of present and future control-plane automations that we want to implement. An impediment is the bespoke nature of current per-task-type async task executors. We think we can substantially improve on this to offer:

• Much higher execution concurrency without holding transactions.
  • While still providing an at-most-one executor guarantee!
• A more flexible programming model for tasks, modeled on (optionally) long-lived coroutines that can spawn other tasks, send messages, yield messages to their parent, and maintain inner state.
• Consistent handling of errors, retries, and task execution leases.
• More focused, type-safe implementations of task executors.

@psFried and I have had many conversations about what this could look like.