workers: job-types, state: applicator: emit task completion events

[akirillo]

This PR introduces a TaskCompletion event type, representing the completion of a task (successful or not). This will be used to index task history, and as such it is basically a wrapper around the HistoricalTask type. This event gets emitted in the state applicator in the same code path where we record historical tasks. As such, we check that the local peer is the executor of the task before emitting the event, to prevent duplicate event emission across the cluster.

Testing

• All unit tests pass (though, with that said, it looks like the new order metadata index tests put each other in deadlock)
• Tested w/ local relayer & event export sidecar, manually inspected events received in SQS to ensure there were no issues in transmission

[joeykraut]

looks good, don't think the ApplicatorQueues type is useful code though