Re-evaulate all PRs when .jcheck/conf changes

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. If the user had a CSR with fixVersion N+1, there will still be a blocker saying:

Change requires a CSR request matching fixVersion N to be approved (needs to be created)

One could also imagine other .jcheck/conf changes that we would like to go into effect immediately, like changing the reviewer requirements.

To solve this, we could have the PullRequestBot (or a separate bot class next to it) check .jcheck/conf for updates and when found, trigger CheckRuns for all open PRs on the repo. This bot can keep a copy of the current .jcheck/conf in memory to compare to. Unfortunately that's not enough to properly handle updates to .jcheck/conf when the bot isn't running. To solve that I think we need to include the contents of .jcheck/conf in the target branch of a PR in the metadata hash. That way, if .jcheck/conf changes in the target branch, it will naturally be cause for re-evaluation. (if .jcheck/conf changes in the PR branch it will already be triggered as that would be a new commit.)