[core split] Layer: concurrent_limit

[Xuanwo]

This issue tracks splitting the layer concurrent_limit out of core as part of the core-split effort.

Tracking issue: #6829

Please follow the steps in the tracking issue strictly:

• keep public API/behavior unchanged;
• move code into the new crate layout described there;
• add/update feature flags and tests as required;
• open a PR and link it to Tracking issues for RFC-6828: Core #6829.

[PsiACE]

/assignme