RFR: Parallelize checking if a test update is needed

[Robin Westberg]

The current TestBot getPeriodItems implementation can starve the scheduler a bit during startup, ensure that it always
returns quickly.

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Commit messages:
 - Parallelize checking if a test update is needed

Changes: https://git.openjdk.java.net/skara/pull/851/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=851&range=00
  Stats: 165 lines in 2 files changed: 106 ins; 53 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/851.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/851/head:pull/851

PR: https://git.openjdk.java.net/skara/pull/851