airframe-http: Simplify HttpFilter hierarchy

[xerial]

No description provided.

[codecov]

Codecov Report

Merging #842 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #842      +/-   ##
==========================================
+ Coverage      81%   81.02%   +0.02%     
==========================================
  Files         226      226              
  Lines        8933     8934       +1     
  Branches      633      638       +5     
==========================================
+ Hits         7236     7239       +3     
+ Misses       1697     1695       -2

Impacted Files	Coverage Δ
...tp/src/main/scala/wvlet/airframe/http/Router.scala	65% <ø> (ø)	⬆️
...ame/http/router/HttpEndpointExecutionContext.scala	95.23% <ø> (ø)	⬆️
...la/wvlet/airframe/http/finagle/FinagleFilter.scala	100% <ø> (ø)	⬆️
...a/wvlet/airframe/http/finagle/FinagleBackend.scala	75% <ø> (-7.76%)	⬇️
...c/main/scala/wvlet/airframe/http/HttpContext.scala	90% <100%> (+15%)	⬆️
...c/main/scala/wvlet/airframe/http/HttpBackend.scala	100% <100%> (ø)	⬆️
...t/airframe/http/router/HttpRequestDispatcher.scala	95.45% <100%> (+0.21%)	⬆️
...rc/main/scala/wvlet/airframe/http/HttpFilter.scala	100% <100%> (+18.18%)	⬆️

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