RFR: 1963: Reduce the frequency of fetching the same issue from IssueProject in CheckRun

[Erik Joelsson]

This patch adds a cache for `IssueTrackerIssue` in `CheckWorkItem`. This will ensure that we only ever fetch issues from the issue project once in each work item. I'm also inlining the rather common `IssueProject` null check wherever possible.

Note that this patch is built on top of #1535 so will go in after that one.

Doing this also fixes a flaw with the current metadata hash logic. At the end of a check run, the metadata hash is updated. The current implementation fetches all issues again at that point, which means that we may store an updated version of an issue, that hasn't been part of the check run, in the hash. With this change, we are guaranteed to store the same issue state as was used in the check run.

-------------

Depends on: https://git.openjdk.org/skara/pull/1535

Commit messages:
 - SKARA-1963

Changes: https://git.openjdk.org/skara/pull/1536/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1536&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1963
  Stats: 28 lines in 2 files changed: 14 ins; 4 del; 10 mod
  Patch: https://git.openjdk.org/skara/pull/1536.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1536/head:pull/1536

PR: https://git.openjdk.org/skara/pull/1536