RocketChatCache: fixes to avoid confusion between URLs on different hosts having the same path
- RocketChatCache: include host name in relative paths to avoid confusion between URLs on different hosts having the same path
The only exception is URLs on the chat server itself: e.g. keep using "avatar/dfaure" rather than "hostname/avatar/dfaure" to minimize disruption.
- RocketChatCache: use full URL for "already in download" set to avoid confusion between URLs on different hosts having the same path