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Whenever certain record is written generation and its digest are stored
in metadata file, and then all previous metadata file(s) are
asynchronously deleted.
When reopening the StorageQueue, metadata file matching the last
sequencing number is used to restore generation and digest for future
writes.
The code does not handle errors yet - missing or corrupt metadata file,
incomplete data file, etc. In the future we will add code to reset
generation id and eliminate last digest.

MockClient in tests makes sure generation_id is the same for all records.
It also verify records own digest; last record digest is not verified
yet, because the test expects some records to be duplicated.

Bug: b:153364303
Bug: b:153659559
Change-Id: I80d8e7f03097bcc4a510f21e0a546876b3104960
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2388716
Commit-Queue: Leonid Baraz <lbaraz@chromium.org>
Reviewed-by: Zach Trudo <zatrudo@google.com>
Cr-Commit-Position: refs/heads/master@{#804486}