Pass :remote_shutdown messages to the backend

[seancribbs]

Based on the documentation, I was expecting my FLAME.Backend implementation to be able to receive the {:remote_shutdown, reason} message so it could do some cleanup, but they were not being passed to my handle_info callback. This remediates that problem.

While I was in there, I got stumped the argument order of maybe_backend_handle_info being switched from the order of handle_info arguments, so I swapped them to be consistent.