persist: add ability to store per-device "numbers"

A number is a 64-bit value stored per-device (or for self if "addr" is
NULL). Basically just like a key, except it is a number.

BUG=chromium:805112
TEST=build
Change-Id: Iee5d9e687a111f5554398abec9ef60ea67b0ca76
2 files changed