Replace caller function with setTimeout since setTimeout allows last arg...

...ument to be omitted.

Technically, if we call `nextTick` with 2 arguments (where the last one is not `0`), this would not be identical, but this *might* improve performance.

Due to the potential regression, we might not want to merge this. Your choice :).
I guess I try to only make changes when there is a clear benefit and I am not sure quite yet what we are trying to improve.  Readability, performance?  Or just personal preference?