RFR: 1937: Re-evaulate all PRs when .jcheck/conf changes [v2]

[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.

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  update

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

Changes:
  - all: https://git.openjdk.org/skara/pull/1531/files
  - new: https://git.openjdk.org/skara/pull/1531/files/48581714..89ff415f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1531&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1531&range=00-01

  Stats: 122 lines in 15 files changed: 66 ins; 41 del; 15 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