PLT-6916: Redesign the jobs package and Jobserver. (#6733)

This commit redesigns the jobserver to be based around an architecture
of "workers", which carry out jobs of a particular type, and "jobs"
which are a unit of work carried by a particular worker. It also
introduces "schedulers" which are responsible for scheduling jobs of a
particular type automatically (jobs can also be scheduled manually when
apropriate).

Workers may be run many times, either in instances of the platform
binary, or the standalone jobserver binary. In any mattermost cluster,
only one instance of platform OR jobserver must run the schedulers. At
the moment this is controlled by a config variable, but in future will
be controlled through the cluster leader election process.

i18n/en.json

@@ -3743,6 +3743,10 @@
     "id": "error.not_found.title",
     "translation": "Page not found"
   },
+  {
+    "id": "jobs.request_cancellation.status.error",
+    "translation": "Could not request cancellation for job that is not in a cancelable state."
+  },
   {
     "id": "manaultesting.get_channel_id.no_found.debug",
     "translation": "Could not find channel: %v, %v possibilities searched"
@@ -5292,24 +5296,24 @@
     "translation": "We couldn't save or update the file info"
   },
   {
-    "id": "store.sql_job_status.delete_by_type.app_error",
-    "translation": "We couldn't delete the job status"
+    "id": "store.sql_job.delete.app_error",
+    "translation": "We couldn't delete the job"
   },
   {
-    "id": "store.sql_job_status.get.app_error",
-    "translation": "We couldn't get the job status"
+    "id": "store.sql_job.get.app_error",
+    "translation": "We couldn't get the job"
   },
   {
-    "id": "store.sql_job_status.get_all.app_error",
-    "translation": "We couldn't get all job statuses"
+    "id": "store.sql_job.get_all.app_error",
+    "translation": "We couldn't get the jobs"
   },
   {
-    "id": "store.sql_job_status.save.app_error",
-    "translation": "We couldn't save the job status"
+    "id": "store.sql_job.save.app_error",
+    "translation": "We couldn't save the job"
   },
   {
-    "id": "store.sql_job_status.update.app_error",
-    "translation": "We couldn't update the job status"
+    "id": "store.sql_job.update.app_error",
+    "translation": "We couldn't update the job"
   },
   {
     "id": "store.sql_license.get.app_error",