Refactor cfg.onFlightState, use atomic bool

[JoTurk]

Description

Simplify handshakeCompletedSuccessfully, use atomic.Bool and use CompareAndSwap in setHandshakeCompletedSuccessfully to avoid potential data race!

Reference issue

Fixes #667

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.63%. Comparing base (f5e908f) to head (806ff2f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
+ Coverage   78.58%   78.63%   +0.04%     
==========================================
  Files         101      101              
  Lines        6831     6828       -3     
==========================================
+ Hits         5368     5369       +1     
+ Misses       1089     1085       -4     
  Partials      374      374              

Flag	Coverage Δ
go	78.65% <100.00%> (+0.04%)	⬆️
wasm	57.11% <100.00%> (-0.02%)	⬇️

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