[IC] Implement leveldb metadata storage and unittests

Initial store for metadata, used for keeping track of which
images need to be garbage collected.

Bug: 878850
Change-Id: I6d55c941c55ca0916196728761c2f9954759fb87
Reviewed-on: https://chromium-review.googlesource.com/1203414
Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591071}
6 files changed