CollectionChanged subscriptions count as reset handling

Consumers of ReactiveCollection that subscribes to CollectionChanged
either directly through and event subscription or indirectly by
subscribing to the Changed observable will now be counted as handling
reset events and won't get a warning.
Skip reset in derived collection constructor

There's no need to do a full reset (and publish events) in the
constructor of derived collections. Doing this reordering makes us
avoid triggering the reset subscription warning in the base collection
that would otherwise occur due to us not having subscribed to events
yet.