Rhino stability fixes

[didimitrie]

Related to CNX-774: Rhino 8 Deep Crashes

Main fix is to ensure that we're executing delicate operations on the main thread:

• purging for second send
• layer creation/deletion

Baking objects in the document is still happening as before.

[linear]

CNX-774 Rhino 8 Deep Crashes

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 8.39%. Comparing base (b2606d2) to head (37dd3eb).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
....RhinoShared/ToHost/Raw/PolylineToHostConverter.cs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             dev    #375      +/-   ##
========================================
- Coverage   8.40%   8.39%   -0.01%     
========================================
  Files        237     237              
  Lines       4690    4691       +1     
  Branches     512     513       +1     
========================================
  Hits         394     394              
- Misses      4280    4281       +1     
  Partials      16      16              

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