Replace absolute cache control with ETag

Replace absolute cache control with ETag

Hard-coding no-store / expires: -1 into the handler prevents
the possibility of caching responses, which we might want for
offline mode. We can use the ETag header to get clients to
always try a network request; if they have connectivity, then
the server will always return an updated response.

Signed-off-by: Dan Scott <dscott@laurentian.ca>