fix ConfigClient data race

[wcvb13]

fix #481

[codecov-commenter]

Codecov Report

Merging #483 (6a93c8b) into master (bcceb19) will decrease coverage by 0.23%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
- Coverage   51.23%   50.99%   -0.24%     
==========================================
  Files          25       25              
  Lines        2100     2102       +2     
==========================================
- Hits         1076     1072       -4     
- Misses        935      941       +6     
  Partials       89       89              

Impacted Files	Coverage Δ
clients/config_client/config_client.go	46.87% <44.44%> (+0.01%)	⬆️
clients/naming_client/beat_reactor.go	77.96% <0.00%> (-6.78%)	⬇️
clients/naming_client/push_receiver.go	30.68% <0.00%> (-3.41%)	⬇️
clients/naming_client/host_reactor.go	60.16% <0.00%> (+1.62%)	⬆️

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