RFR: 1256: Provide a common method to get the corresponding csr issue

[Guoxiong Li]

Hi all,

This patch adds the common methods `Issue#linksWithRelationships`, `Issue#csrLink` and `Issue#csrIssue` and refactors the code to reduce the loop nested statements by using these common methods.

All the existing tests passed.

Best Regards,
-- Guoxiong

-------------

Commit messages:
 - 1256: Provide a common method to get the corresponding csr issue

Changes: https://git.openjdk.java.net/skara/pull/1248/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1248&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1256
  Stats: 265 lines in 8 files changed: 140 ins; 69 del; 56 mod
  Patch: https://git.openjdk.java.net/skara/pull/1248.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1248/head:pull/1248

PR: https://git.openjdk.java.net/skara/pull/1248