Serialize now waits for the scheduler to be completed before returning

[adamhathcock]

Trying to stop occasional indeterminate tests 😬

Cancellation for download should be snappier as the token is listened to and the stream happens asap with ResponseHeadersRead

[codecov]

Codecov Report

Attention: Patch coverage is 74.04580% with 34 lines in your changes missing coverage. Please review.

Project coverage is 70.79%. Comparing base (988599f) to head (7f1eef5).

Files with missing lines	Patch %	Lines
...le.Sdk/Serialisation/V2/SerializeProcessFactory.cs	0.00%	9 Missing ⚠️
...c/Speckle.Sdk/Api/Operations/Operations.Receive.cs	0.00%	8 Missing ⚠️
src/Speckle.Sdk/Api/Operations/Operations.Send.cs	0.00%	8 Missing ⚠️
...peckle.Sdk/Serialisation/V2/ServerObjectManager.cs	0.00%	8 Missing ⚠️
...ckle.Sdk.Serialization.Tests/SerializationTests.cs	95.45%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              dev     #225       +/-   ##
===========================================
+ Coverage   59.58%   70.79%   +11.20%     
===========================================
  Files         262      278       +16     
  Lines       10460    11205      +745     
  Branches     1105     1108        +3     
===========================================
+ Hits         6233     7933     +1700     
+ Misses       3964     2918     -1046     
- Partials      263      354       +91     

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[adamhathcock]

I think the empty message ensures all messages are complete and prevents the unstable tests