Keep track of commit times for each blob.

Also, add a commit cool-off period such that any new commits
of the same data blob from the same committer will be treated
as if they occurred at the earlier commit time. This is to
ensure apps don't keep on committing the same blob and prevent
deletion of it.

Bug: 151378266
Test: atest --test-mapping apex/blobstore
Change-Id: I44b3fddbc465b7d6f00f40b299b91d3d84740989