RFR: 1590: Convert PullRequestBot to use new poller

[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.

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Depends on: https://git.openjdk.org/skara/pull/1369

Commit messages:
 - SKARA-1590

Changes: https://git.openjdk.org/skara/pull/1371/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1371&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1590
  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