fix: add commander stopping sync mechanism

[rogercoll]

Resolves #256

Test that generates the data race:

func TestNewSupervisor(t *testing.T) {
...

	supervisor, err := NewSupervisor(&internal.NopLogger{})
	assert.NoError(t, err)

	supervisor.Shutdown()
}

It looks like the commander struct is started in a go routine when the NewSupervisor is created. The data race happens when the commander is stopped (supervisor.Shutdown()) while being initalized (Start).

The fix adds a synchronization mechanism to prevent the commander being started if it is already being stopped.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.29%. Comparing base (2ecac8d) to head (8f99ffa).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #352   +/-   ##
=======================================
  Coverage   78.29%   78.29%           
=======================================
  Files          25       25           
  Lines        2386     2386           
=======================================
  Hits         1868     1868           
  Misses        410      410           
  Partials      108      108           

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Comments addressed.

[tigrannajaryan]

Thanks for the fix @rogercoll