Refactor to use a new class CompletingFutureContribution

[garydgregory]

• Reduces boilerplate when the same pattern is used

[ok2c]

@garydgregory Looks good. Please re-apply with the latest style checks.

[ok2c]

@garydgregory According to the latest japicmp this change-set broke backward (compile) compatibility.

[ERROR] Failed to execute goal com.github.siom79.japicmp:japicmp-maven-plugin:0.23.0:cmp (default) on project httpcore5: There is at least one incompatibility: org.apache.hc.core5.concurrent.FutureContribution:CLASS_GENERIC_TEMPLATE_CHANGED -> [Help 1]
[ERROR] 

[ok2c]

@garydgregory I already have a fix for the problem. I can adjust the change-set if it is fine with you.

[garydgregory]

Hi @ok2c
I am ok with breaking source compatibility but obviously not binary compatibility. If you feel source compatibility is important in this case, then, yeah, some changes are needed and we should consider having the build break when that happens.

[ok2c]

@garydgregory I do not think breakage of compile compatibility is warranted here. Once hc-parent is upgraded a newer version of japicmp will no longer let the build to pass in such cases.