Revise default ScrollSubrange support for QueryDsl and QBE DataFetcher's

[rstoyanchev]

Currently, we accept a default ScrollSubrange to use as a fallback. However, as discussed under #900, this makes it challenging to support scrolling forward from the beginning as well as backward from the end when a position is not specified. We need to allow configuring separately default position for forward vs backward scrolling, and apply those after the user input is processed.

Currently, with offset positions we can only support scrolling from the beginning via ScrollPosition.offset(). With keyset positions we support. both using either ScrollPosition.keyset() or the same with its reverse() method called.