Fix watermarker default time / LevelDB key ordering bug.

This fixes part 2) of https://github.com/prometheus/prometheus/issues/367
(uninitialized time.Time mapping to a higher LevelDB key than "normal"
timestamps).

Change-Id: Ib079974110a7b7c4757948f81fc47d3d29ae43c9
Merge "Fix watermarker default time / LevelDB key ordering bug."