cluster: replace forEach with for..of in lib/internal/cluster/ directory

[jerome-benoit]

No description provided.

[MrHBS]

Out of curiosity why this change?

[jerome-benoit]

Out of curiosity why this change?

Coding style consistency.
There might also have performance implications at using implicitly iterator on collections (no boundary check, less memory footprint, ...), especially on large collection. forEach is probably using already iterator under the hood, with for...of it's explicit and does not depend on an helper implementation.