Enforce document timestamp requirements for updates in backend

The document API has long since had a special field for update operations
where an optional expected _existing_ backend timestamp can be specified,
and where the update should only go through iff there is a timestamp
match.

This has been supported on the distributor all along, but only when
write-repair is taking place (i.e. rarely), but the actual backend
support has been lacking. No one has complained yet since this is
very much not an advertised feature, but if we want to e.g. use this
feature for improvements to batch updates we should ensure that it
works as expected.

With this commit, a non-zero "old timestamp" field is cross-checked
against the existing document, and the update is only applied if the
actual and expected timestamps match.

0 files changed, 0 insertions, 0 deletions