Avoid using whenComplete(); use handle()

[trustin]

Motivation:

As we discovered in #1440, whenComplete() is harmful for performance.

Modifications:

• Use handle() instead of whenComplete()

Result:

• Potentially less unnecessary instantiation of CompletionException.

[codecov]

Codecov Report

Merging #1596 into master will increase coverage by 0.02%.
The diff coverage is 80%.

@@             Coverage Diff              @@
##             master    #1596      +/-   ##
============================================
+ Coverage     72.84%   72.87%   +0.02%     
+ Complexity     7576     7575       -1     
============================================
  Files           699      699              
  Lines         30459    30459              
  Branches       3712     3712              
============================================
+ Hits          22188    22197       +9     
+ Misses         6362     6353       -9     
  Partials       1909     1909

Impacted Files	Coverage Δ	Complexity Δ
.../web/reactive/ArmeriaReactiveWebServerFactory.java	57.89% <100%> (ø)	28 <0> (ø)	⬇️
.../reactive/ArmeriaHttpClientResponseSubscriber.java	61.97% <66.66%> (ø)	12 <0> (ø)	⬇️
.../linecorp/armeria/client/retrofit2/PipeBuffer.java	79.06% <0%> (-4.66%)	6% <0%> (ø)
...corp/armeria/common/stream/FixedStreamMessage.java	85.71% <0%> (-3.58%)	20% <0%> (-2%)
.../armeria/client/endpoint/dns/DnsEndpointGroup.java	82.97% <0%> (-1.07%)	13% <0%> (-1%)
...common/stream/AbstractStreamMessageDuplicator.java	76.79% <0%> (+0.49%)	9% <0%> (ø)	⬇️
...rp/armeria/common/stream/DefaultStreamMessage.java	91.42% <0%> (+1.42%)	54% <0%> (+1%)	⬆️
...om/linecorp/armeria/client/HttpSessionHandler.java	61.2% <0%> (+1.72%)	29% <0%> (+1%)	⬆️
...armeria/client/HttpClientPipelineConfigurator.java	73.06% <0%> (+2.69%)	36% <0%> (ø)	⬇️

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