RFR: 1256: Use stream api to simplify the csr issue lookup [v3]

[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

Guoxiong Li has updated the pull request incrementally with one additional commit since the last revision:

  Improve the encapsulation and use the methods of the 'Optional' instead of direct null check

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/1248/files
  - new: https://git.openjdk.java.net/skara/pull/1248/files/2f640bd7..5d7e353f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1248&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1248&range=01-02

  Stats: 27 lines in 3 files changed: 11 ins; 4 del; 12 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