outposts: rewrite state logic, use cache to expire old channels, support multiple instances

passbook/outposts/migrations/0007_remove_outpost_channels.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.1.2 on 2020-10-14 08:32
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("passbook_outposts", "0006_auto_20201003_2239"),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name="outpost",
+            name="channels",
+        ),
+    ]