Break the relationship between writers and session/subscription

[marcschier]

To better support UNS and aggregation servers that are optimized for low number of subscriptions with large number of monitored items and to support large number of writers (tag writers) we must separate the subscription management from the writer concept, and split monitored item notifications to individual writers. The current model of using a topic context breaks because metadata collection is at the subscription level, it should be at the writer level. The simple way would be to make the subscription "writer aware". Pulling the writer concept out and handling somewhere else would be also possible, current POC though shows the work is large and impactful.