Regard in-memory series as new.

This commit ensures that series that exist only in-memory and not
on-disk are not regarded as too old for operation exclusion.
Code Review: !Before -> After.
Code Review: Swap ordering of watermark getting.

A test for Julius.
Code Review: Swap ordering of elements.
Merge pull request #311 from prometheus/fix/watermarking/on-first-write

Ensure new metrics are watermarked early.