User-facing core API

Jobs are defined as maps with the following attributes:
  :job/id - Required String uniquely identifying a job
  :job/type - Keyword type name of this job
  :job/status - Keyword, one of #{:unstarted :started :finished :failed :aborted}
  :job/args - Optional arguments to the job (must be serializable as EDN)

Jobs that have been run (or attempted) by the system may also have the following attributes:
  :job/heartbeat - Integer UNIX timestamp of the last time a worker processing
                   this job marked itself alive
  :job/failure - Map containing information about the failure of a job

Graphs are defined as Loom digraphs where every node is a valid Job.

:monitor-shutdown - Boolean, whether to shutdown the system when the heartbeat
                    monitor encounters an error (avoids running in a degraded state)
                    Default: true

Functions for reading Overseer configuration. Many configuration values are
optional, and Overseer should 'just work' out of the box. The entire set
of configuration options are:

  :store
    :adapter - Required String backend store type to connect to. One of datomic, mysql, h2, sqlite
    :config - options to configure the selected adapter
      If Datomic:
        - map of :uri, required connection String, ex: "datomic:free://localhost:4334/overseer"
      If MySQL/H2:
        - clojure.java.jdbc "database spec"
          - map of :dbtype, :dbname and other options as needed, such as :user, :password, :host, :port
          - map of driver :classname, :subprotocol, :hostname, :port, :subname, :user, :password
          - map of :connection-uri String option passed directly to driver
          - map of :name and :environment keys for JNDI connection
          - Can also just be a String JDBC URI
          See http://clojure-doc.org/articles/ecosystem/java_jdbc/home.html#setting-up-a-data-source

  :sentry - Optional map configuring the Sentry error backend
    :dsn - String DSN to use

  :detector-sleep-time - Optional; How long to sleep between ready job detector runs in ms
                         (default: 2000)

  :sleep-time - Optional; How long to sleep in ms if the job queue is empty (default: 10000)

  :heartbeat - map of optional attributes to configure worker heartbeating
    :sleep-time - How long to sleep in ms before persisting heartbeat (per-worker)
                  (default: 60000)
    :tolerance - How many heartbeats can fail before job is considered dead
               to be reset by a monitor (default: 5)

    :monitor-shutdown - Boolean, whether to shutdown the system when the heartbeat
                        monitor encounters an error (avoids running in a degraded state)
                        Default: true