don't try to ancient pack already large storages

[jeffwashington]

Problem

Account state continues to grow.
We'd like to turn off rent exempt rewrites.
At that point, we accumulate old append vecs.
We append or pack those into 'ancient' append vecs.
The packing algorithm can go off the rails in certain testing scenarios.
Note that the ancient algorithm is currently 'append'. This code does not affect 'append' at all.
The packing algorithm is the future. Packing is enabled with --accounts-db-create-ancient-storage-packed

Summary of Changes

Apply some limits to what the packing algorithm will attempt.
In this case, we skip trying to pack storages that are not shrink candidates but are already close to ideal size.

This limit is a stop gap to prevent machines from running oom by trying to combine too many and too large append vecs in one pass (by not flushing the write cache most likely or by loading pubkeys into memory for billions of accounts).

Fixes #

[brooksprumo]

lgtm

I think if you rebase on latest master it'll pull in a fix for the CI checks dependency issue.