Make default scheduler remove cancelled tasks.

[avakhrenev]

Should prevent leaking timeouts.

[rossabaker]

I think this method is only on ScheduledThreadPoolExecutor and you'll need a pattern match for it to compile. 👍 to the concept.

[avakhrenev]

@rossabaker right, thank you.

[codecov-io]

Codecov Report

Merging #568 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #568      +/-   ##
==========================================
- Coverage    88.8%   88.75%   -0.05%     
==========================================
  Files          77       77              
  Lines        2143     2143              
  Branches      129      126       -3     
==========================================
- Hits         1903     1902       -1     
- Misses        240      241       +1

[rossabaker]

Thanks!