Make the settle after a delivered walk a setting, two minutes by default (ADR 0505 §3)

modules/messenger/module.json

@@ -140,6 +140,14 @@
       "maximum": 1000,
       "trusted": false,
       "default": 100
+    },
+    "settle-seconds": {
+      "type": "number",
+      "description": "seconds a delivery that ended well still holds the messages about what it changed, so a problem that passes as the delivery settles is not sent (ADR 0505 §3); 0 holds nothing after the end.",
+      "minimum": 0,
+      "maximum": 3600,
+      "trusted": false,
+      "default": 120
     }
   },
   "resources": [
@@ -163,7 +171,7 @@
       "type": "file",
       "path": "${dir:state}/settings.json",
       "mode": "0600",
-      "takes-config": ["time-zone", "desktop-machines", "only-what-needs-me", "messages-per-hour"],
+      "takes-config": ["time-zone", "desktop-machines", "only-what-needs-me", "messages-per-hour", "settle-seconds"],
       "merge": "json",
       "template": "files/settings.json",
       "owner": "messenger"