Add a missing boundary check for Range requests

Fixes #1323
Spotted & fix by:	gquintard
This test didn't actually use persistent storage because the
"shortlived" test puts the object in Transient.

Patch by:	gquintard
Fixes		#1387
Fix an assert to test that the object has "useful lifetime" which
is not the same as a positive TTL any more.
Fix the "shortlived" test to look a the sum of ttl + grace + keep
rather than just the ttl.

Fixes		#1268
Spotted by:	daghf
Partial fix by:	gquintard
Fix sphinx warning
Start work on a "What's New" section
Add missing anchor
When we get a bogus reply from a backend, for instance with too many
headers, we need to purge all traces of it, before we construct our
503 response, otherwise we risk failing to set important headers,
such as "Connection: close".

Fixes #416
(again)