Adjust offset based pagination to be exclusive of reference position

[rstoyanchev]

The Relay spec expects scrolling to be exclusive of the referenced position. However, offset based scrolling is inclusive of the reference position and also always scrolls forward.

For forward scrolling we need make an adjustment to advance by 1. We have not been doing that from the beginning, and have been returning items from (and including) the reference position.

For backward scrolling we need to advance back by the count and switch to forward. We had been doing that until 1.2.4 when a change was made based on the report in #840 to advance by 1 extra, which is in fact incorrect. We'll need to go back and investigate #840 again once more, but the adjustment needs to be made to correct the behavior.

[rstoyanchev]

There is now a 1.2.5-SNASPHOT with the changes in https://repo.spring.io/snapshot.