Fiber loops in Renderer and BigPipe are never suspended

Problem/Motivation

Fiber loops in Renderer and BigPipe are never suspended due to $iterations being reset at every iteration of the while loop

Steps to reproduce

1. Create a placeholder callback that call Fiber::suspend at least twice
2. Place breakpoint in the BigPipe.php:577 where it suspends to allow higher code to execute
3. Iterations are always 0 so Fiber::suspend never called

It does not really affect anything yet because there are no higher fibers yet.

Proposed resolution

Move the $iterations declaration out of the while loops.

Remaining tasks

• Move $iteration declarations
• Provide tests if possible/needed

User interface changes

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API changes

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Data model changes

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Release notes snippet

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