Improve style of pg_upgrade task callback functions.

I wanted to avoid adjusting this code too much when converting
these tasks to use the new parallelization framework (see commit
40e2e5e92b), which is why this is being done as a follow-up commit.
These stylistic adjustments result in fewer lines of code and fewer
levels of indentation in some places.

While at it, add names to the UpgradeTaskSlotState enum and the
UpgradeTaskSlot struct.  I'm not aware of any established project
policy in this area, but let's at least be consistent within the
same file.

Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/ZunW7XHLd2uTts4f%40nathan

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