etcdserver: watch stream got closed if one watch request is not permitted

[polyrabbit]

This pr fixes #11708

The issue is easy to fix, but hard to reproduce. It happens only when:

1. Client successfully established a watch stream
2. Client issues another watch request which results in a permission denied response
3. After that, every watch request that reuses the same stream will hang forever

@jingyih @gyuho @xiang90

[codecov-io]

Codecov Report

Merging #11754 into master will decrease coverage by 0.81%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #11754      +/-   ##
==========================================
- Coverage   66.55%   65.74%   -0.82%     
==========================================
  Files         403      403              
  Lines       36859    36860       +1     
==========================================
- Hits        24531    24233     -298     
- Misses      10841    11112     +271     
- Partials     1487     1515      +28     

Impacted Files	Coverage Δ
etcdserver/api/v3rpc/watch.go	82.16% <0.00%> (-2.40%)	⬇️
client/client.go	43.13% <0.00%> (-40.85%)	⬇️
client/keys.go	68.34% <0.00%> (-23.12%)	⬇️
pkg/transport/timeout_conn.go	60.00% <0.00%> (-20.00%)	⬇️
auth/store.go	52.38% <0.00%> (-9.56%)	⬇️
pkg/logutil/zap_grpc.go	47.61% <0.00%> (-4.77%)	⬇️
proxy/grpcproxy/watcher.go	89.79% <0.00%> (-4.09%)	⬇️
clientv3/leasing/cache.go	87.77% <0.00%> (-3.89%)	⬇️
etcdserver/api/v3rpc/member.go	87.09% <0.00%> (-3.23%)	⬇️
clientv3/leasing/txn.go	88.09% <0.00%> (-3.18%)	⬇️
... and 19 more

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[xiang90]

LGTM. Defer to @jingyih

[jingyih]

LGTM