Stop backdating SXGs by 1 day

It's confusing DX (especially when combined with amp-script max-age; see #395), and also less useful for caching.

I think this is dependent on better clock skew support in the major consumer (Google; tracked in http://b/133525127), or else this change would cause a higher NEL error rate (redirect fallback).

No files or tests are named. Start by tracing where SXG timestamps are generated, then review the linked clock-skew dependency and the interaction with AMP-script max-age; done requires removing the one-day backdating without increasing redirect fallback or NEL errors.

Written by the indexing model from the issue text.