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Implemented a cache in form of a [key, StrikeData] map for set or
retrieved values to avoid hitting the disk repeatedly for the same
card.

Bug: 884717
Change-Id: I5e8a5936a54f30a64a919a97314cd04d3874b2a2
Reviewed-on: https://chromium-review.googlesource.com/c/1249130
Commit-Queue: Anne Lim <annelim@google.com>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Jared Saul <jsaul@google.com>
Cr-Commit-Position: refs/heads/master@{#597235}