describe different persistance vs model update strategies

doc/sync-updates.md

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+# persistance vs model update of a room
+
+## persist first, return update object, update model with update object
+ - 
+## update model first, return update object, persist with update object
+ - not all models exist at all times (timeline only when room is "open"),
+ 	so model to create timeline update object might not exist for persistence need
+
+## persist, update, each only based on sync data (independent of each other)
+ - possible inconsistency between syncing and loading from storage as they are different code paths
+ + storage code remains very simple and focussed
+
+## updating model and persisting in one go
+ - if updating model needs to do anything async, it needs to postpone it or the txn will be closed
+
+## persist first, read from storage to update model
+ + guaranteed consistency between what is on screen and in storage
+ - slower as we need to reread what was just synced every time (big accounts with frequent updates)