RFR: 1590: Convert PullRequestBot to use new poller [v3]

[Erik Joelsson]

> With the new poller introduced in [SKARA-1565](https://bugs.openjdk.org/browse/SKARA-1565), we can now start converting more bots to use it. This issue covers the PullRequestBot. I'm posting the PR now already mostly to demonstrate of well the new poller fits in.

Erik Joelsson has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 17 commits:

 - Merge branch 'master' into SKARA-1590-poller-pr-bot
 - Merge branch 'SKARA-1565-pr-poll' into SKARA-1590-poller-pr-bot
 - Adjusted some comments
 - Fix tests
 - Use new PullRequest API
 - Revert CSRBot, moving it to separate change
 - Merge branch 'master' into SKARA-1565-pr-poll
 - SKARA-1590
 - Adding log message
 - Merge branch 'SKARA-1589-test-refactor' into SKARA-1565-pr-poll
 - ... and 7 more: https://git.openjdk.org/skara/compare/ca8d09a2...a0f88a58

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Changes: https://git.openjdk.org/skara/pull/1371/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1371&range=02
  Stats: 37 lines in 1 file changed: 1 ins; 22 del; 14 mod
  Patch: https://git.openjdk.org/skara/pull/1371.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1371/head:pull/1371

PR: https://git.openjdk.org/skara/pull/1371