Improves asyncutils/Debounce

[farazdagi]

What type of PR is this?

Other/Refactoring/Minor fixing

What does this PR do? Why is it needed?

• Better implementation of context cancelling: context cancelling channel was listened only in inner loop, making it impossible to get out of the function if no incoming events are sent (in this PR: we can create debouncer, never send any event to it, and on context cancellation -- debouncer stops and returns).
• time.After aggregated timer channels, and depending on the rate of incoming events it might be a problem (too many resources to garbage collect). I've changed routine to use explicit time.Timer.C instead -- so at the moment only a single timer channel is open at a time. And when we need to reset (on every incoming event), previous timer is stopped, and new timer is created (which is exactly what happened when we used time.After() with additional benefit of stopping the previous timer before creating a new one).
• Fixes race condition in test: if you run tests many times over, eventually you will step into failed test (when timer is almost triggered, but not fast enough and test assertions already is testing the result).
• Tests all the scenarios fully (no events, cancelling using inner loop, cancelling using outer loop, single handler invocation, multiple invocations -- to make sure that debouncer's outer loop works correctly).

Which issues(s) does this PR fix?

Other notes for review

• cc @rauljordan

[codecov]

Codecov Report

Merging #7189 into master will decrease coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7189      +/-   ##
==========================================
- Coverage   61.77%   61.65%   -0.13%     
==========================================
  Files         410      410              
  Lines       32678    32556     -122     
==========================================
- Hits        20188    20073     -115     
- Misses       9613     9617       +4     
+ Partials     2877     2866      -11     

[rauljordan]

Learned a lot from reading this code, thanks @farazdagi !