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The prefetch store SQL table was missing retry counter columns for some of the
pipeline operations that needed those counters. This CL adds those columns and
also changes their current naming pattern. A new store test and a mock item
generator are also added to verify that saving and loading a prefetch item is
consistent.

Note: Some of the functionality being used only in tests here will support the
upcoming addition of a pipeline task to deal with expired entries.

Bug: 701939
Change-Id: Iaa26975fb17ec8ab979925613b4af743ef176cd6
Reviewed-on: https://chromium-review.googlesource.com/585334
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Dmitry Titov <dimich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491279}