A 304 "Not Modified" should update the time to when we next expect a modification
In legacy/trac#20499 (moved), we discovered that when a 304 "Not Modified" is received, relays try too hard when 09a0f2d0 is reverted, and don't try enough when it is applied.
Instead, we should retry when we next expect the document to be modified. And if we don't know, we should retry on some sensible schedule.