feat(event-switch): add a graceful stop to the EventSwitch

[fthouraud]

It allows the EventSwitch to be closed only after all events have been read then received by all the receivers.

The setState method has been improved. The loop over all the states is now skipped if the new state is either Aborted or Emitted.

I've also:

• cleaned up the code
• improved documentation
• fixed the test for massive events (it was only using 1 receiver instead of 10)

[codecov-io]

Codecov Report

Merging #5 into master will increase coverage by 1.56%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   75.81%   77.38%   +1.56%     
==========================================
  Files           7        7              
  Lines         612      650      +38     
==========================================
+ Hits          464      503      +39     
  Misses        107      107              
+ Partials       41       40       -1     

Impacted Files	Coverage Δ
godim.go	70.93% <70.00%> (-0.87%)	⬇️
event.go	76.11% <89.21%> (+5.14%)	⬆️

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