RFR: Add toString methods to all WorkItems

[Erik Helin]

Hi all,

please review this small patch that adds proper `toString` implementations to
all classes implementing `WorkItem` (to get better logging).

Thanks,
Erik

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Commits:
 - 1f8ffb73: Add toString methods to all WorkItems

Changes: https://git.openjdk.java.net/skara/pull/403/files
 Webrev: https://webrevs.openjdk.java.net/skara/403/webrev.00
  Stats: 24 lines in 8 files changed: 20 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/403.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/403/head:pull/403

PR: https://git.openjdk.java.net/skara/pull/403