RFR: csr bot: use pull request cache

[Erik Helin]

Hi all,

please review this patch that makes the CSR bot use the pull request cache. The
CSR bot now has two phases, the first phase simply checks if any updated has
gotten the "csr" label. The addition of a "csr" label can only happen by
comments being added to the PR, so the `PullRequestCache` will return these PRs
are requiring an update.

The second phase then loops through all pull requests that have the CSR label
and check if the CSR request has been resolved.

Testing:
- `make test` passes on Linux x64

Thanks,
Erik

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Commits:
 - 623db4bf: csr bot: use pull request cache

Changes: https://git.openjdk.java.net/skara/pull/451/files
 Webrev: https://webrevs.openjdk.java.net/skara/451/webrev.00
  Stats: 43 lines in 1 file changed: 20 ins; 0 del; 23 mod
  Patch: https://git.openjdk.java.net/skara/pull/451.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/451/head:pull/451

PR: https://git.openjdk.java.net/skara/pull/451