Job lifecycle management: the JobEnqueue and WorkerExecution surfaces.

The system map passed to all functions must contain:
  :pool      - HikariCP DataSource (from the database component)
  :validator - PayloadValidator (from the validation component)

Typical usage:

  ;; Build the system
  (def sys {:pool pool :validator validator})

  ;; Enqueue
  (add-job sys "send-email" {:to "x@y.com"} {:queue-name "email"})

  ;; Worker loop
  (let [jobs (get-jobs sys worker-id {:batch-size 5})]
    (doseq [job jobs]
      (try
        (run-task! job)
        (complete-jobs sys worker-id [job] elapsed-ms)
        (catch Exception e
          (fail-jobs sys worker-id [{:job job :error e}] elapsed-ms)))))

Malli schemas for job-manager component.