SE-0417: Add globalConcurrentExecutor and allow (any TaskExecutor)? preference consistently (#2271)

* SE-0417: Add DefaultConcurrentExecutor and remove to `some` params

* Update 0417-task-executor-preference.md

Co-authored-by: Remy Demarest <[email protected]>

* optional executor argument and global concurrent executor

* further clarify the handling of NIL

* Update proposals/0417-task-executor-preference.md

Co-authored-by: Xiaodi Wu <[email protected]>

* Apply suggestions from code review

Co-authored-by: John McCall <[email protected]>
Co-authored-by: Xiaodi Wu <[email protected]>

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Co-authored-by: Remy Demarest <[email protected]>
Co-authored-by: Xiaodi Wu <[email protected]>
Co-authored-by: John McCall <[email protected]>

Author: 4 people