RFR: Cache labels in GitHubPullRequest

[Erik Helin]

Hi all,

please review this pull request that introduces a simple label cache for `GitHubPullRequest`. Since a `GitHubPullRequest` instance can't be accessed by multiple threads it is safe to use a very simple cache mechanism. The only calls that can mutate labels on a PR are `addLabel` and `removeLabel`. A user on GitHub could of course add a label, but we make no guarantees that a `GitHubPullRequest` instance reflects what happens on GitHub, code wanting to ensure they have the latest state must create a new `GitHubPullRequest` instance.

Thanks,
Erik

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Commits:
 - 114b5d69: Cache labels in GitHubPullRequest

Changes: https://git.openjdk.java.net/skara/pull/294/files
 Webrev: https://webrevs.openjdk.java.net/skara/294/webrev.00
  Stats: 18 lines in 1 file changed: 13 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/294.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/294/head:pull/294

PR: https://git.openjdk.java.net/skara/pull/294