restores for state at seconds(now-forDuration)

[owen-d]

This fixes a bug where we were incorrectly restoring for state. Loki will try to synthesize a historical ALERTS_FOR_STATE upon first evaluation (i.e. after restart) by evaluating the rule at both now-for_duration and now, which should allow it to recover after restarts, etc. This PR fixes a bug where the returned sample value in this time series was the millisecond timestamp of now, rather than the unix seconds representation of now-for_duration. I tested this locally with the following rule, which now successfully tries to alert before the 10m duration is up.

groups:
  - name: should_fire
    interval: 10s
    rules:
      - alert: absent_err
        expr: 'absent_over_time({job="foo"}[1m])'
        for: 10m

[dannykopping]

LGTM