RFR: 2297: CSRs for OpenJDK 8u backports not being picked up

[Zhao Song]

Currently, the version matching logic in `Backports::findClosetIssue` and `Backports::findIssue` differs, causing the skara bot fail to find CSRs for some issues.

In this patch, I'm trying to unify the version matching logic between the two methods. I couldn't find a good way to reuse the previous methods(`Backports::matchVersion`, `Backports::matchOptPoolVersion`, `Backports::matchPoolVersion`, `Backports::matchScratchVersion`), so I changed the interface of the methods.

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Commit messages:
 - update comment
 - SKARA-2297

Changes: https://git.openjdk.org/skara/pull/1661/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1661&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2297
  Stats: 59 lines in 2 files changed: 5 ins; 23 del; 31 mod
  Patch: https://git.openjdk.org/skara/pull/1661.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1661/head:pull/1661

PR: https://git.openjdk.org/skara/pull/1661