RFR: Use separate timestamps for updated at and expires at

[Robin Westberg]

The updatedAt timestamp comes from the server, so use a separate timestamp to determine cache expiration.

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Commit messages:
 - Use separate timestamps for updated at (which comes from the server) and the expires at (which is local)

Changes: https://git.openjdk.java.net/skara/pull/861/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=861&range=00
  Stats: 11 lines in 1 file changed: 7 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/861.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/861/head:pull/861

PR: https://git.openjdk.java.net/skara/pull/861