buffet: Organize storage keys by relative lifetime

This has no functional effect but makes things a little more readable.

BUG=None
TEST=Unittests

Change-Id: Iaeb1536e3aeb622a899c91e58b0fd9b3528ce0a2
Reviewed-on: https://chromium-review.googlesource.com/247502
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>