Integrated: Cache check expiration times

[Robin Westberg]

Caching the check expiration times avoids a per-PR network request.

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Commit messages:
 - Cache check expiration times

Changes: https://git.openjdk.java.net/skara/pull/867/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=867&range=00
  Stats: 34 lines in 4 files changed: 16 ins; 12 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/867.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/867/head:pull/867

PR: https://git.openjdk.java.net/skara/pull/867