In implementing the library to consume the service-types data, it became
clear that the behavior could be much more consistent across
implementations if we set cache-control headers. This allows a combined
ETag and time-based approach, so that the data will only be fetched if
it has a stale etag, but it will only be checked for staleness once a
week. Since the data in question is expected to change only rarely, and
then only in additive ways, this should allow pervassive use of the data
without significant cost to the API consumer.
Change-Id: I6de3c79e22fdea9bf70fd725447ca7141af80b50
fc98399aa4