RFR: 1937: Re-evaulate all PRs when .jcheck/conf changes

[Zhao Song]

As Erik said in the issue: "When we bump the version in .jcheck/conf from N to N+1 in the jdk master branch, PRs with CSRs are often not updated to reflect this."

This root cause of this is that changing of .jcheck/conf in target ref of the pr will not be able to trigger checkWorkItem for this pr.

In this patch,
1. The prBot will monitor the changes to the .jcheck/conf file in any branch(except for PR branches).
2. If the prBot detects an update to the .jcheck/conf file in a branch, it will trigger a checkWorkItem for all open PRs whose target ref is the branch where the .jcheck/conf file was updated.
3. To solve the problem related with bot restarting, the contents of the .jcheck/conf file in the target branch of a PR will be included in the metadata hash.

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Commit messages:
 - update
 - update
 - fix tests
 - fix test
 - update
 - Merge branch 'master' into SKARA-1937
 - SKARA=1937

Changes: https://git.openjdk.org/skara/pull/1531/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1531&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1937
  Stats: 133 lines in 9 files changed: 130 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/skara/pull/1531.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1531/head:pull/1531

PR: https://git.openjdk.org/skara/pull/1531