*: support CORS for v3 HTTP requests

[gyuho]

And some clean up

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2e1e619). Click here to learn what that means.
The diff coverage is 77.45%.

@@            Coverage Diff            @@
##             master    #9490   +/-   ##
=========================================
  Coverage          ?   72.72%           
=========================================
  Files             ?      366           
  Lines             ?    30998           
  Branches          ?        0           
=========================================
  Hits              ?    22542           
  Misses            ?     6834           
  Partials          ?     1622

Impacted Files	Coverage Δ
etcdserver/server.go	80.24% <ø> (ø)
etcdserver/config.go	79.8% <ø> (ø)
embed/etcd.go	69.64% <100%> (ø)
etcdmain/config.go	83.91% <100%> (ø)
etcdmain/etcd.go	50% <100%> (ø)
embed/config.go	60.07% <25%> (ø)
etcdserver/server_access_control.go	50% <50%> (ø)
embed/serve.go	63.42% <64.51%> (ø)
pkg/flags/unique_urls.go	82.35% <82.35%> (ø)
pkg/flags/unique_strings.go	83.33% <83.33%> (ø)

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