Cancellationmanager revamp

[adamhathcock]

Cancellation manager shouldn't dispose when cancelling
Force removal when send/receive is done

[codecov]

Codecov Report

Attention: Patch coverage is 97.29730% with 2 lines in your changes missing coverage. Please review.

Project coverage is 18.79%. Comparing base (184c53c) to head (83ea845).

Files with missing lines	Patch %	Lines
...nectors.Common/Cancellation/CancellationManager.cs	90.90%	0 Missing and 1 partial ⚠️
...Speckle.Connectors.Common/ContainerRegistration.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #527      +/-   ##
==========================================
+ Coverage   17.31%   18.79%   +1.48%     
==========================================
  Files         239      240       +1     
  Lines        4818     4883      +65     
  Branches      579      579              
==========================================
+ Hits          834      918      +84     
+ Misses       3954     3934      -20     
- Partials       30       31       +1     

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