DATAMONGO-1057 - Fix SliceExecution skipping elements.

We now directly set the offset to use instead of reading it from the used pageable. This asserts that every single element is read from the store.
Prior to this change the altered pageSize lead to an unintended increase of the number of elements to skip.

Original pull request: #226.
DATAMONGO-1057 - Polishing.

Slightly tweaked the changes in SlicedExecution to simplify the implementation. We now apply the given pageable but tweak the limit the query uses to peek into the next page.

Original pull request: #226.