Fix the data race in NetworkCurl

[mykhailo-kuchma]

Fixed the RequestHandle initialization; it has to be protected with a mutex
Fixed the condition in cancel event handling to avoid canceling wrong requests

Relates-To: DATASDK-69

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.16%. Comparing base (884a201) to head (c76ac1a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1595      +/-   ##
==========================================
- Coverage   80.16%   80.16%   -0.00%     
==========================================
  Files         347      347              
  Lines       13756    13755       -1     
  Branches     1472     1470       -2     
==========================================
- Hits        11027    11026       -1     
  Misses       2133     2133              
  Partials      596      596              

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