*: clean up for Go 1.8+

[gyuho]

ref. #6174

[xiang90]

lgtm when ci passes. thank you!

[gyuho]

Blocked on etcd-ci-ppc64 tests.

@heyitsanthony
Can we ask the maintainer to change its Go runtime?

[codecov-io]

Codecov Report

Merging #7737 into master will increase coverage by 0.18%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7737      +/-   ##
==========================================
+ Coverage   75.75%   75.93%   +0.18%     
==========================================
  Files         331      331              
  Lines       26058    26038      -20     
==========================================
+ Hits        19739    19772      +33     
+ Misses       4894     4842      -52     
+ Partials     1425     1424       -1

Impacted Files	Coverage Δ
pkg/transport/listener.go	80.5% <ø> (-2.83%)	⬇️
embed/serve.go	74.33% <100%> (ø)	⬆️
proxy/grpcproxy/register.go	69.44% <0%> (-13.89%)	⬇️
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
proxy/grpcproxy/watcher.go	89.58% <0%> (-4.17%)	⬇️
etcdserver/api/v3rpc/util.go	55.55% <0%> (-2.78%)	⬇️
pkg/schedule/schedule.go	81.94% <0%> (-1.39%)	⬇️
clientv3/balancer.go	94.15% <0%> (-1.17%)	⬇️
rafthttp/transport.go	81.92% <0%> (-1.13%)	⬇️
etcdserver/server.go	79.3% <0%> (-0.58%)	⬇️
... and 13 more

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[xiang90]

merging. ppc64 is ok now.