kn/results skip performance

[nielsenko]

This makes skip a simple integer addition, and defer handle construction as long as possible.

• Fetch handle in _RealmResultsIterator.current lazily (it may never be called for a given index)
• Support efficient skip on RealmResults

This came about due to https://www.mongodb.com/community/forums/t/do-relationship-properties-get-populated-during-tolist/242260.

skip is useful if you wan't to do pagination for various reasons, even though realm doesn't require it. In this case to support an existing data access layer previously build on another DB.

Fixes: #1392

[nirinchev]

Should we also implement efficient skip for list as well?

[nielsenko]

Should we also implement efficient skip for list as well?

Given that ManagedRealmList implements List using a ListMixin (which now a days is equal to ListBase) it actually already uses an efficient implementation for skip.

  Iterable<E> skip(int count) => SubListIterable<E>(this, count, null);

SubListIterable does the right arithmetics for us.