etcdmain: best effort detection of self pointing in tcp proxy

[xiang90]

Best effort approach to detect if the tcp gateway points to itself. Or the gateway might direct traffic to itself and cause an endless traffic loop.

[codecov-io]

Codecov Report

Merging #11845 into master will decrease coverage by 0.23%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #11845      +/-   ##
==========================================
- Coverage   66.03%   65.79%   -0.24%     
==========================================
  Files         403      403              
  Lines       36993    37021      +28     
==========================================
- Hits        24428    24358      -70     
- Misses      11054    11148      +94     
- Partials     1511     1515       +4     

Impacted Files	Coverage Δ
etcdmain/gateway.go	25.00% <0.00%> (-8.88%)	⬇️
auth/options.go	35.00% <0.00%> (-57.50%)	⬇️
clientv3/balancer/utils.go	53.84% <0.00%> (-46.16%)	⬇️
pkg/transport/timeout_conn.go	60.00% <0.00%> (-20.00%)	⬇️
auth/range_perm_cache.go	54.28% <0.00%> (-14.29%)	⬇️
pkg/netutil/netutil.go	61.47% <0.00%> (-6.56%)	⬇️
auth/store.go	52.38% <0.00%> (-6.47%)	⬇️
clientv3/namespace/watch.go	87.87% <0.00%> (-6.07%)	⬇️
raft/tracker/inflights.go	91.83% <0.00%> (-4.09%)	⬇️
etcdserver/util.go	95.23% <0.00%> (-3.58%)	⬇️
... and 25 more

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[gyuho]

lgtm thx