add example app notify-stress

[bill1600]

Add example test app to examples directory.
The app "notify-stress" sends notifications to parodus.
./notify-stress -n 500000
will send 500000 messages to parodus
./notify-stress -n 0
will send messages to parodus continuously till you ctl-C.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.44%. Comparing base (fcef57c) to head (43642af).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #49   +/-   ##
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  Coverage   62.44%   62.44%           
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  Files           3        3           
  Lines         639      639           
  Branches      131      131           
=======================================
  Hits          399      399           
  Misses        165      165           
  Partials       75       75           

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