payloadLifespan/payloadLifetime and other name variants unified to Time To Live naming.

code/MessengerApi.Db.Contracts/Entities/Message.cs

@@ -20,6 +20,6 @@ namespace MessengerApi.Db.Entities
 
         public string Payload { get; set; }
 
-        public int? PayloadLifespanInSeconds { get; set; }
+        public int? TimeToLiveInSeconds { get; set; }
     }
 }