Move scrubber outside the store package

Glance's scrubber was created under the store package because it is
intended to clean up images from stores when `delayed_delete` is
enabled. However, it doesn't really belong there.

Soft delete is a Glance feature that allows images to be deleted
asynchronously as opposed of blocking clients requests during the image
deletion. Therefore, it needs to be store agnostic and live within
Glance's codebase.

Partially-Implements blueprint: create-store-package

Change-Id: I25ddbe1f0f1191f33d814dbae5788da7d511773a
Merge "Move scrubber outside the store package"