clientv3: modify lock type.

[johncming]

from #10580

1. Concurrent access is possible in func (c *Client) Endpoints() (eps []string) , so mutex required.
2. remove naked return.

[codecov-io]

Codecov Report

Merging #10595 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #10595      +/-   ##
==========================================
+ Coverage    71.6%   71.66%   +0.06%     
==========================================
  Files         393      393              
  Lines       36571    36573       +2     
==========================================
+ Hits        26185    26211      +26     
+ Misses       8548     8527      -21     
+ Partials     1838     1835       -3

Impacted Files	Coverage Δ
clientv3/client.go	80.22% <100%> (+0.11%)	⬆️
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
etcdserver/api/v3rpc/lease.go	72.72% <0%> (-4.55%)	⬇️
etcdserver/v2_server.go	80.76% <0%> (-3.85%)	⬇️
lease/leasehttp/http.go	62.77% <0%> (-2.92%)	⬇️
etcdctl/ctlv3/command/printer_simple.go	72.48% <0%> (-1.35%)	⬇️
etcdserver/api/v2http/client.go	85.51% <0%> (-1.21%)	⬇️
proxy/grpcproxy/watch.go	88.55% <0%> (-1.21%)	⬇️
etcdserver/api/rafthttp/stream.go	79.39% <0%> (-0.43%)	⬇️
etcdserver/server.go	74.39% <0%> (-0.15%)	⬇️
... and 12 more

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[gyuho]

@johncming Can we investigate the test failures?

[johncming]

@johncming Can we investigate the test failures?

ok, i will try it.

[xiang90]

lgtm