Fix calculation of packed slots minimum when calculating accounts to combine

[dmakarov]

Problem

In ancient pack, we can encounter storages which can't be safely moved to new slots. One example is when a storage contains an alive account whose refcount > 1. In that case, we can only move the alive account to a slot that is newer than the current slot. Otherwise, we risk moving the alive account to a slot older than the currently dead account. If we restarted from a snapshot, we would have the wrong alive version of an account.
When determining whether individual storages can be packed, when we encounter a storage with a multi ref account and we determine we can't pack it this time, we have now decreased the # of alive bytes we'll be writing. As a result, we now reduce the # required ideal packed storages. As a result, this makes it easier for multi ref storages we encounter later in this pack to succeed.

Summary of Changes

If we determine a storage cannot be packed, reduce the # alive bytes so we can reduce the # ideal storages we'll be creating. This allows the algorithm to make progress.

[jeffwashington]

lgtm

[HaoranYi]

looks correct to me.

With the tighter bounds of target_slots, we will have more chance to pack.