Calculating etags takes time. It seems we should be able to cache an etag
with the item. Maybe an automatic _etag property added tot hey hyperdb base object like
the created or activity property. (IIRC the activity property is already used when
checking if-modified-since.) To prevent loops, the _etag would not be included in
the etag calculation and would be automatically updated by the db code when anything
in the item changed.

The web front end could just pull the _etag directly rather than retrieving
the whole item and calculating the etag. This would speed up conditional gets.
It could still append/prepend data to the etag to produce a different tag based
on content-encoding etc.

See also: issue2551356 - Add etag header when If-Modified-Since GET request returns not-
modified (304)