restart-server: Check if service is running before restart, vs start.

In some instances (e.g. during upgrades) we run `restart-server` and
not `start-server`, even though we expect the server to most likely
already be stopped.  `supervisorctl restart servicename` if the
service is stopped produces the perhaps-alarming message:

```
restart-server: Restarting servicename
servicename: ERROR (not running)
servicename: started
```

This may cause operators to worry that something is broken, when it is
not.

Check if the service is already running, and switch from "restart" to
"start" in cases where it is not.

The race condition here is safe -- if the service transitions from
stopped to started between the check and the `start` call, it will
merely output:
```
servicename: ERROR (already started)
```
...and continue, as that has exit status 0.

If the service transitions from started to stopped between the check
and the `restart` call, we are merely back in the current case, where
it outputs:
```
servicename: ERROR (not running)
servicename: started
```

In none of these cases does a call to "restart" fail to result in the
service being stopped and then started.

scripts/restart-server

@@ -98,6 +98,14 @@ aux_services = list_supervisor_processes(["go-camo", "smokescreen"], only_runnin
 if aux_services:
     subprocess.check_call(["supervisorctl", "start", *aux_services])
 
+
+def restart_or_start(service: str) -> None:
+    our_verb = action
+    if our_verb == "restart" and len(list_supervisor_processes([service], only_running=True)) == 0:
+        our_verb = "start"
+    subprocess.check_call(["supervisorctl", our_verb, service])
+
+
 if action == "restart" and len(workers) > 0:
     if args.less_graceful:
         # The less graceful form stops every worker now; we start them
@@ -111,7 +119,7 @@ if action == "restart" and len(workers) > 0:
         # requires multiple `supervisorctl restart` calls.
         for worker in workers:
             logging.info("Restarting %s", worker)
-            subprocess.check_call(["supervisorctl", "restart", worker])
+            restart_or_start(worker)
 
 if has_application_server():
     # Next, we restart the Tornado processes sequentially, in order to
@@ -130,12 +138,10 @@ if has_application_server():
             # supervisord group where if any individual process is slow to
             # stop, the whole bundle stays stopped for an extended time.
             logging.info("%s Tornado process on port %s", verbing, p)
-            subprocess.check_call(
-                ["supervisorctl", action, f"zulip-tornado:zulip-tornado-port-{p}"]
-            )
+            restart_or_start(f"zulip-tornado:zulip-tornado-port-{p}")
     else:
         logging.info("%s Tornado process", verbing)
-        subprocess.check_call(["supervisorctl", action, "zulip-tornado:*"])
+        restart_or_start("zulip-tornado:*")
 
     # Finally, restart the Django uWSGI processes.
     if (
@@ -160,7 +166,7 @@ if has_application_server():
             subprocess.check_call(["supervisorctl", "start", "zulip-django"])
     else:
         logging.info("%s django server", verbing)
-        subprocess.check_call(["supervisorctl", action, "zulip-django"])
+        restart_or_start("zulip-django")
 
     using_sso = subprocess.check_output(["./scripts/get-django-setting", "USING_APACHE_SSO"])
     if using_sso.strip() == b"True":