Improve update collection mechanism for sync v3

[bwindels]

• BaseMappedList updater should use UpdateAction to be able to replace the existing mapped value (or update, or swallow do update)
• TemplateView needs to resubscribe on update() and support replacing the view model with another.
• If sync v3 updates perf seems problematic, either of these may be a solution:
  • properly implement the reset event on collections
  • consider a bulk update mechanism for observable collections.
• other lazylistviews have shown some friction by not having random access. We could add a method to create an iterator at a given index.