`BufferedSeriesIterator` and `MemoizedSeriesIterator` use a method
called `Values` for exactly the purpose for which all other iterators
of the same kind use a method called `At`. That alone is confusing,
but on top of that, the `Values` method only returns a single sample,
not multiple values. I assume the naming has historical reasons. This
commit makes it more consistent. It is now easier to read, and now
`BufferedSeriesIterator` and `MemoizedSeriesIterator` implement
`chunkenc.Iterator` like many other iterators, too.
Signed-off-by: beorn7 <beorn@grafana.com>
