Include deletion processor for long-tail values.

This commit extracts the model.Values truncation behavior into the actual
tiered storage, which uses it and behaves in a peculiar way—notably the
retention of previous elements if the chunk were to ever go empty.  This is
done to enable interpolation between sparse sample values in the evaluation
cycle.  Nothing necessarily new here—just an extraction.

Now, the model.Values TruncateBefore functionality would do what a user
would expect without any surprises, which is required for the
DeletionProcessor, which may decide to split a large chunk in two if it
determines that the chunk contains the cut-off time.
Merge pull request #234 from prometheus/feature/storage/long-tail-deletion

Include deletion processor for long-tail values.