AutoGetCollection now has operators making it behave like a smart pointer.

With lock free reads it can be dangerous to call AutoGetCollection::getCollection and cache the returned pointer as a subsequent call to AutoGetCollection::getWritableCollection can invalidate the previous pointer.

We should use operator-> and operator* on the AutoGetCollection when possible to encapsulate the Collection lifetime within the AutoGetCollection.

While doing this we should also consider changing interfaces that pass along const Collection* to be const Collection&. That would make it clean to get the necessary type by just dereferencing the AutoGetCollection instance.

Also while we are in here: consider renaming AutoGetCollection::getCollection() to AutoGetCollection::get() to make the interface more consistent with unique and shared pointer. As it is a Collection RAII type, the default return should be a Collection.