2.1.1

Fixed — on the null cache driver, 2.1.0 answered every delivery with 409

2.1.0 made the replay claim a single conditional Cache::add(), which is right
for the race it fixes and wrong about one thing: a false from add() means
either "the key is already there" or "this store keeps nothing at all", and on
the null driver only the second is ever true. Every delivery to an endpoint
with replay protection on was therefore rejected as a duplicate of nothing — a
cache setting turning a working endpoint off, which is worse than the missing
guard, and more likely now that new endpoints have the guard on by default.

One read on the rejection path tells the two apart. Stores that actually
remember are unaffected, and the claim is still one conditional write.

Affected: 2.1.0 only, and only with CACHE_STORE=null.