store accounts async

[palmerlao]

This undoes the account prefetcher, which really implemented prefetch and async storage at the same time in the account loader, in favor of implementing just async storage in accounts DB.

testing

I crossed two epoch boundaries 3 times each with no incident:

./mithril verify-range \
...
--snapshot /mnt/nvme/snapshots-backup/snapshot-392161343-CV9cWMjcEk6gkh6ZRr5iFRFeLBmreauHJrCywJuVYL2F.tar.zst \
--incremental-snapshot /mnt/nvme/snapshots-backup/incremental-snapshot-392161343-392253715-HZPo3grHCoskYDWmdHX6XXLxBxX66hSEozLwHGNeZggU.tar.zst \
--num-slots 3000 \
...

./mithril verify-range \
...
--snapshot /mnt/nvme/snapshots-backup/snapshot-392663585-8FNmbEu9yZDTjNZbDKY7xxjA584Nch8DMeg53wkzLHu3.tar.zst \
--incremental-snapshot /mnt/nvme/snapshots-backup/incremental-snapshot-392663585-392686279-G2YJnh1A4rES4McB9ceQTFy8cEY1oJL24AXTTnjEj6Ep.tar.zst \
...
--num-slots 2250 \
...

I also ctrl C'd/resumed replay a few times.

performance

I ran this on f4d5dec and c48e6d3 (dev before prefetcher):

./mithril verify-range \
--load-from-snapshot \
--snapshot /mnt/mithril-ledger/snapshots/snapshot-395273312-BMG9gUFWoQVGSh53MQyXC36hHbTHEJzpBhnz2GZPUYb2.tar.zst \
--incremental-snapshot /mnt/mithril-ledger/snapshots/incremental-snapshot-395273312-395368707-9vNrVHXxLqdScJnjEomEjVdWmzAArF6FmaAMkhhH4ytV.tar.zst \
--accounts-path /mnt/mithril-accounts \
--num-slots 1000 \
--rpc=REDACTED \
--config config.toml

The slot replay duration improvement is more modest, only ~10ms. Probably due to LoadBlockAccounts still being on critical path and added coordination. However, this design decouples prefetch and async storage. It also could evolve to keeping modified accounts in memory until a slot is committed. For example, the list could become a tree of uncommitted slot modifications, and we don't have to invoke the store worker on those modifications until we know some subtree is confirmed.