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Fixes a serious bug in the StorageQueue found with the stress test.
When records are enqueued fast (e.g. in multiple threads) it can cause
the following: the sequence ids will always be strictly monotonic, but
last record digest may refer to a wrong record:
(1) Record1 gets last digest of previous record
(2) Record2 gets digest of Record1
(3) Record2 gets seq id=1 and is written to the file
(4) Record1 gets seq_id=2 and is written to the file.
Upon uploading, server will get Record2[seq=1] uploaded first,
Record1[seq=2] second, but last record digest from Record1 will not
match digest of Record2 - it will match record before Record1.
Add stress test too.

Bug: b:177439641
Change-Id: I5b7e425ea6aa0f30b96248dcaebb22562c085613
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2641569Reviewed-by: Zach Trudo <zatrudo@google.com>
Commit-Queue: Leonid Baraz <lbaraz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846352}