Object Store implementations should set cache headers

[mhutchinson]

The AWS and GCP implementations serve their tiles, entry bundles, and checkpoint from object storage. Neither of the implementations set cache headers on this data. I haven't queried to see what the default headers these artifacts are served with, but the observation is that in the current situation it is almost certain that checkpoints are being served with the same cache headers as tile data.

In #398 the MySQL implementation was updated to set no caching on checkpoints, and to set a long-lived cache header on the entry bundles. This approach should be taken for the object store artifacts too.