[ISSUE #123]Fix config client cachemap initialization logic

[binbin0325]

fix cachemap had multiple initializations， causes the cacheData to be emptied.#123

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@170bb4e). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #125   +/-   ##
=========================================
  Coverage          ?   66.77%           
=========================================
  Files             ?       19           
  Lines             ?     1192           
  Branches          ?        0           
=========================================
  Hits              ?      796           
  Misses            ?      326           
  Partials          ?       70           

Impacted Files	Coverage Δ
clients/config_client/config_client.go	59.44% <100.00%> (ø)

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[lzp0412]

LGTM