picker: replace picker with V2Picker ，picker is deprecated

[Murphylu1993]

Please read https://github.com/etcd-io/etcd/blob/master/CONTRIBUTING.md#contribution-flow.

[codecov-io]

Codecov Report

Merging #12384 into master will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #12384      +/-   ##
==========================================
- Coverage   63.28%   63.20%   -0.08%     
==========================================
  Files         448      448              
  Lines       62342    62348       +6     
==========================================
- Hits        39451    39410      -41     
- Misses      18325    18372      +47     
  Partials     4566     4566              

Impacted Files	Coverage Δ
clientv3/balancer/picker/picker.go	50.00% <ø> (ø)
clientv3/balancer/balancer.go	86.66% <100.00%> (+0.30%)	⬆️
clientv3/balancer/picker/err.go	100.00% <100.00%> (ø)
clientv3/balancer/picker/roundrobin_balanced.go	100.00% <100.00%> (ø)
pkg/netutil/netutil.go	61.47% <0.00%> (-7.38%)	⬇️
pkg/fileutil/purge.go	69.56% <0.00%> (-6.53%)	⬇️
api/etcdserverpb/raft_internal_stringer.go	32.25% <0.00%> (-6.46%)	⬇️
clientv3/namespace/watch.go	87.87% <0.00%> (-6.07%)	⬇️
etcdctl/ctlv3/command/lease_command.go	65.34% <0.00%> (-5.95%)	⬇️
etcdserver/api/v3rpc/interceptor.go	69.18% <0.00%> (-3.49%)	⬇️
... and 23 more

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[Murphylu1993]

/retest

[ptabor]

Thank you for the contribution. Could you, please elaborate/link more in commit & PR description on the purpose of the change.

Looking at the current state of https://godoc.org/google.golang.org/grpc/balancer#Picker, in the current version its called "Picker" again (but conforms to your interface). Is there a plan to rename it back after update of the grpc library ?

[Murphylu1993]

Thank you for the contribution. Could you, please elaborate/link more in commit & PR description on the purpose of the change.

Looking at the current state of https://godoc.org/google.golang.org/grpc/balancer#Picker, in the current version its called "Picker" again (but conforms to your interface). Is there a plan to rename it back after update of the grpc library ?

done

[ptabor]

Thank you for the contribution. Could you, please elaborate/link more in commit & PR description on the purpose of the change.
Looking at the current state of https://godoc.org/google.golang.org/grpc/balancer#Picker, in the current version its called "Picker" again (but conforms to your interface). Is there a plan to rename it back after update of the grpc library ?

done

I see the title changed. But what about lack of V2 picker in the current state of library: https://godoc.org/google.golang.org/grpc/balancer#Picker

[ptabor]

This PR overlaps with broader: #12398 2859e9f