Improve finalizer safety for Session, SshChannel, and SftpSession

[JamesWrigley]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 84.84848% with 5 lines in your changes missing coverage. Please review.

Project coverage is 83.29%. Comparing base (c4120e3) to head (1d6c838).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/session.jl	75.00%	2 Missing ⚠️
src/sftp.jl	89.47%	2 Missing ⚠️
src/channel.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
- Coverage   83.31%   83.29%   -0.02%     
==========================================
  Files          10       10              
  Lines        1954     1976      +22     
==========================================
+ Hits         1628     1646      +18     
- Misses        326      330       +4     

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