RFR: storage: cache deserialized values in FileStorage

[Erik Helin]

Hi all,

please review this patch that adds caching of the deserialized value to `FileStorage`. There is no point in
deserializing the same `String` over and over again, the deserialized value will remain the same.

Testing:
- [x] Added a new unit test
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - storage: cache deserialized values in FileStorage

Changes: https://git.openjdk.java.net/skara/pull/656/files
 Webrev: https://webrevs.openjdk.java.net/skara/656/webrev.00
  Stats: 54 lines in 2 files changed: 53 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/656.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/656/head:pull/656

PR: https://git.openjdk.java.net/skara/pull/656