perf: bound replay memory and improve account-storage reclaim

[smcio]

We noticed that sustained bursts of blocks rewriting large accounts demonstrated two pretty important resource management problems. These were:

• slot count-only fold batching could result in retention of enough account versions to exhaust RAM
• obsolete account data accumulated on disk because rewind references pinned entire files and compaction did not revisit candidates quickly enough. This could result in disk exhaustion

These changes fix both of these problems.