[PATCH] Fix autovacuum freeze bug where weight can lower a table's freeze score

Hi hackers,

In the autovacuum freeze scores added by d7965d65fc5, raising a freeze
weight can make a table's score smaller instead of larger. When a table
gets old, its score is raised to a power to grow fast, but a larger weight
makes this start while the score is still below one, where a power makes it
smaller. So a bigger weight can push a table near wraparound down the list.
The safety net still works; only the order changes.

The attached patch raises the score to a power only when that makes it
bigger, with a test. New in v19, so no backport needed.

Thanks,
Shihao