Dates were kept in each process's rewrite cache, so replicas sharing a
database and storage dated independently: one dated a document, upstream
changed in that second, and another gave the new document the same date,
so a client holding it got 304 and kept the old document.
- Date from metadata_dates instead of process memory: a package's date is
the second it started serving its current representation, recorded once
for every replica
- Switch a package to another representation only in a second later than
its recorded date, and serve it undated until then
- Date nothing in the second the process started, so dates clients hold
from before this change, or from the process it replaced, stay older
- Serve no date ahead of the local clock, recorded by a replica whose clock
is ahead
- Date rewrites without the rewrite cache too; the cache keeps only ETags
- Add two-replica, pre-fix date and clock skew regression tests
