ssl_handshake is now handled by worker threads, and not by cheroot main thread

[bmourgues]

❓ What kind of change does this PR introduce?
cheroot accepts new requests while performing ssl handshake on some connections

• 🐞 bug fix
• 🐣 feature
• 📋 docs update
• 📋 tests/coverage improvement
• 📋 refactoring
• 💥 other

📋 What is the related issue number (starting with #)
#748

Resolves #748

❓ What is the current behavior? (You can also link to an open issue here)

❓ What is the new behavior (if this is a feature change)?

📋 Other information:
pre-commit is ok, except for 2 files I did not change (_compat.py and errors.py)
base branch was main/10.0.x

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• I have squashed related commits together after
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• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
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This change is 

[webknjaz]

@bmourgues hi, the process is to always send patches against main with an option to backport to older branches. Although, I wasn't really planning to keep the v10.x.x stream alive, so a backport is rather unnecessary.

[bmourgues]

Hello, I made a new PR against main #750 , you can discard this one.

About code, ssl_wrap could have been directly done in get method of threadpool, but I don't know if this would be ok with pyOpenSSL
if self.server.ssl_adapter is not None and not isinstance(conn.socket, SSLSocket): ... do_wrapping

[webknjaz]

Thanks. Though, you could've updated the base branch in the PR and rebased on that to keep it. I'll close this PR, so there's no confusion between the two.

P.S. Try to use code highlighting when referencing snippets.