buffet: Refactor async operations. Use less of lambdas and keep the logic in methods. BUG=None TEST=cros_workon_make buffet --test&&manual Change-Id: I6bc3bddaa69763b029af6960630b1bd1e5f5c30e Reviewed-on: https://chromium-review.googlesource.com/225960 Reviewed-by: Anton Muhin <antonm@chromium.org> Commit-Queue: Anton Muhin <antonm@chromium.org> Tested-by: Anton Muhin <antonm@chromium.org>