Close Session when closing SCPEngine or SFTPEngine

[evigeant]

This PR fixes an issue where the SessionChannel created for an SCP or SFTP command was not closed explicitely. As such, doing many transfers in a connection would accumulate open channels and this can cause issues if the number of open channels is limited by the server.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 69.06%. Comparing base (03f8b22) to head (699f6b1).
Report is 4 commits behind head on master.

❗ Current head 699f6b1 differs from pull request most recent head 90df54c. Consider uploading reports for the commit 90df54c to get more accurate results

Files	Patch %	Lines
...main/java/net/schmizz/sshj/xfer/scp/SCPEngine.java	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #926      +/-   ##
============================================
+ Coverage     68.96%   69.06%   +0.09%     
+ Complexity     1448     1446       -2     
============================================
  Files           208      208              
  Lines          7602     7607       +5     
  Branches        658      659       +1     
============================================
+ Hits           5243     5254      +11     
+ Misses         2009     2002       -7     
- Partials        350      351       +1     

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