Add some null-proofing in DEBUG_REQ, and supply arguments for all the format

Add some null-proofing in DEBUG_REQ, and supply arguments for all the format
fields (!) while we're at it.

Bullet-proof expired_request to see if we can figure out why we sometimes see
NULL derefs during timeout on dev.

Make the cleanup-from-sending-list stuff use D_ERROR, so that we can see it even
if the client crashes afterwards.  (Just playing a hunch.)