Allow the caller to query for resources updated within a certain
time range. Changes the storage engine API as well as the web
API.
Also includes a fix for the event date range query, which was
broken when both start and end values were used.
Change-Id: Ic482ea647ad07553a6621ba84ed76cf10d534652
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>