Rename `next` and `prev` to `getOlder` and `getNewer` respectively

[morleyzhi]

From @fnando:

Right now, it uses next() and prev(), but this is tricky because it depends on sorting; by default, prev() fetches newer records, next() fetch older records. If you sort as asc, then is the opposite. What if use actually use older() and newer() instead? Then the semantics would always be the same in both cases.