Update to Kotlin 2.1.20 and minor refactoring

[devcrocod]

• update Kotlin version to 2.1.20
• use atomics from kotlin.concurrent
• refactor lock in StrdioServerTransport, using Channel
• remove the atomicfu plugin

How Has This Been Tested?

• unit test
• run a custom mcp server with Claude via stdio

Breaking Changes

No breaking changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

default atomics: https://kotlinlang.org/docs/whatsnew2120.html#common-atomic-types

[Copilot]

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

[e5l]

lgtm

[Stream29]

It's just a question, but why you deprecate atomicfu?

It have performance issues or compatibility issues?

I'm curious.

[devcrocod]

@Stream29
I didn't deprecate atomicfu
In Kotlin 2.1.20, atomic types were added to std kotlin.concurrent.atomics
This allowed us to remove the dependency on the atomicfu plugin, since in this code we only use common atomic types.
You can read more about it here