Separate RequestContextAwareFuture version overrides into mrjar…

[anuraaga]

… source sets.

Now that we are using mrjar for version-specific implementations, we can use it for the recent change for completablefuture to support Java 9, and future ones to support Java 12.

Advantages

• Can use other Java 9 APIs when implementing a Java 9 API (e.g., super.completeAsync)
• Maybe some small performance by not needing e.g., maybeMakeContextAware, supplyAsync for completion of current future
• Possible readability improvement by being able to use @Override and otherwise IDE-awareness of the API level

Disadvantage

• If user shades armeria into non-mrjar and runs on newer Java, newer APIs wouldn't function correctly

I think shading is advanced enough that the disadvantage is ok but wondering what everyone thinks.

[codecov]

Codecov Report

Merging #2127 into master will increase coverage by 0.11%.
The diff coverage is 2.22%.

@@             Coverage Diff              @@
##             master    #2127      +/-   ##
============================================
+ Coverage     73.65%   73.76%   +0.11%     
+ Complexity     9518     9498      -20     
============================================
  Files           835      834       -1     
  Lines         36691    36586     -105     
  Branches       4528     4526       -2     
============================================
- Hits          27024    26987      -37     
+ Misses         7332     7269      -63     
+ Partials       2335     2330       -5

Impacted Files	Coverage Δ	Complexity Δ
...linecorp/armeria/internal/JavaVersionSpecific.java	60% <0%> (-6.67%)	3 <0> (ø)
...internal/RequestContextAwareCompletableFuture.java	0% <0%> (ø)	0 <0> (?)
...inecorp/armeria/common/AbstractRequestContext.java	69.09% <100%> (-5.46%)	22 <1> (-2)
.../linecorp/armeria/internal/Http2ObjectEncoder.java	71.42% <0%> (-8.58%)	12% <0%> (-1%)
...linecorp/armeria/common/HttpRequestDuplicator.java	80.95% <0%> (-4.05%)	8% <0%> (ø)
...meria/internal/AbstractHttp2ConnectionHandler.java	93.33% <0%> (-3.34%)	13% <0%> (-1%)
...om/linecorp/armeria/server/jetty/JettyService.java	63.05% <0%> (-2.96%)	23% <0%> (-1%)
...inecorp/armeria/server/grpc/ArmeriaServerCall.java	86.61% <0%> (-1.58%)	81% <0%> (-1%)
...necorp/armeria/client/HeapBasedEventLoopState.java	92.78% <0%> (-1.04%)	31% <0%> (-1%)
... and 11 more

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[trustin]

Thanks, @anuraaga!

[minwoox]

👍

[ikhoon]

Thanks! @anuraaga
I left some nits :-)