limit ancient pack budget for shrinking

[jeffwashington]

Problem

Testing with skipping rewrites and ancient packing have resulted in a steady state where we have too many ancient storages that need to be 'shrunk'. These already large, ancient storages dominate the budget of a maximum of 10 ancient append vecs of ideal size. The result is that packing fails to make progress on keeping a limit on the number of ancient storages/roots.

Summary of Changes

Limit the shrink budget based on the overall limit of ideal storages to produce.

Fixes #

[brooksprumo]

The new threshold being the min of either (some percentage of the alive bytes) or (some percentage of the max ideal size) makes sense.

[brooksprumo]

All of CI has passed except for the one broken downstream projects test. Since this PR doesn't touch anything program-related, I think it is safe to merge.

#2208 will fix the CI. An alternative would be to rebase and pick up this change once the PR is merged, but then this PR will need to go through all of CI again.