zilencer: Lock the RemoteZulipServer row when inserting data.

This does not ensure that we do not mix data from multiple servers
sharing a UUID -- if one has more `RemoteRealmCount` rows,
and the other has more `RemoteInstalltionCount` rows, the end result
will still be some rows from each server, across the two tables.

It does ensure that we will not alternate rows between two servers
if both requests are processed at the same time.

It also causes submissions to be all-or-nothing in the event of
integrity errors.  This is not necessarily beneficial, as forward
progress is generally useful -- but the integrity errors are resolved
in the subsequent commit.

zilencer/views.py

@@ -617,6 +617,7 @@ class InstallationCountDataForAnalytics(BaseModel):
 
 
 @typed_endpoint
+@transaction.atomic
 def remote_server_post_analytics(
     request: HttpRequest,
     server: RemoteZulipServer,
@@ -627,6 +628,10 @@ def remote_server_post_analytics(
     realms: Optional[Json[List[RealmDataForAnalytics]]] = None,
     version: Optional[Json[str]] = None,
 ) -> HttpResponse:
+    # Lock the server, preventing this from racing with other
+    # duplicate submissions of the data
+    server = RemoteZulipServer.objects.select_for_update().get(id=server.id)
+
     if version is not None:
         version = version[0 : RemoteZulipServer.VERSION_MAX_LENGTH]
     if version != server.last_version: