airframe-http: Http client interface clean up

[xerial]

Follow-up work of #2230

[codecov]

Codecov Report

Merging #2231 (0f7a5c5) into master (5613aa3) will not change coverage.
The diff coverage is 81.81%.

@@           Coverage Diff           @@
##           master    #2231   +/-   ##
=======================================
  Coverage   82.61%   82.61%           
=======================================
  Files         313      314    +1     
  Lines       12428    12428           
  Branches      773      771    -2     
=======================================
  Hits        10268    10268           
  Misses       2160     2160           

Impacted Files	Coverage Δ
...frame/http/codegen/client/RPCClientGenerator.scala	100.00% <ø> (ø)
...n/scala/wvlet/airframe/http/HttpClientConfig.scala	47.61% <ø> (ø)
...scala/wvlet/airframe/http/client/HttpClients.scala	72.82% <ø> (+2.11%)	⬆️
...ala/wvlet/airframe/http/client/ClientFactory.scala	42.85% <42.85%> (ø)
...et/airframe/http/client/URLConnectionChannel.scala	86.84% <91.30%> (ø)
...wvlet/airframe/http/client/JavaClientChannel.scala	96.96% <100.00%> (ø)
...cala/wvlet/airframe/http/HttpClientException.scala	77.21% <0.00%> (-1.27%)	⬇️
...me-log/src/main/scala/wvlet/log/io/StopWatch.scala	81.05% <0.00%> (+0.20%)	⬆️
...frame-rx/src/main/scala/wvlet/airframe/rx/Rx.scala	85.18% <0.00%> (+1.23%)	⬆️

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