Follow GS implementation of skip: in BtreePlusReadStream

[brunobuzzi]

The current implementation fault objects into memory until reach the desired position. The assignment form := btreePlusReadStream next. fault the object into memory.
Check:
https://github.com/brunobuzzi/OrbeonPersistenceLayer/blob/master/repository/OrbeonPersistenceApi.package/OrbeonSummaryStream.class/instance/fetchIndexedFormsInstancesOn.from.to..st#L13-L22
Check:
http://forum.world.st/Position-method-for-BtreePlusReadStream-classes-td5118055.html