Cloud backend: change the ChunkCache's replacement policy to LRU. fixes #1885

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
Cloud backend: suppress unhandled TaskStopped exceptions from FileBodyProducer.

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
Cloud backend: log URLs for HTTP responses, as well as requests.

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>