Add Tree concurrency tests

[humdrum]

What this PR does / why we need it:

• Add Tree concurrency tests yorkie-js-sdk#792
• remove syncShutdownGracefully() in Client.
  • syncShutdownGracefully() caused hangs when performing Tree concurrent tests.
  • close connections are performed with rpcClient.channel.close() so, It's redundant.
• Create some deepcopy() function to classes.
  • The initial Tree root is used repeatedly with the Tree concurrent test.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 718 lines in your changes are missing coverage. Please review.

Project coverage is 38.89%. Comparing base (97ed0c8) to head (3018468).

Files	Patch %	Lines
Tests/Integration/TreeConcurrencyTests.swift	0.00%	707 Missing ⚠️
Sources/Document/Change/ChangeContext.swift	0.00%	5 Missing ⚠️
Sources/Document/Json/JSONTree.swift	0.00%	5 Missing ⚠️
Sources/Core/Client.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #169      +/-   ##
==========================================
- Coverage   40.42%   38.89%   -1.54%     
==========================================
  Files         104      105       +1     
  Lines       18179    18895     +716     
==========================================
  Hits         7349     7349              
- Misses      10830    11546     +716     

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