trie/triedb/pathdb: fix initialization flaws

[holiman]

Fix for #28713

Original problem was caused by #28595 . In this PR, we made it so that as soon as we start to sync, we delete the root of the disk layer. That is not wrong per se, but, another part of the code uses the "presence of the root" as an init-check for the pathdb. And, since the init-check now failed, it tried to initialize it, which failed since a sync was already ongoing.

Impact: after a state-sync has begun, if the node is restarted, it will refuse to restart, with the error message: Fatal: Failed to register the Ethereum service: waiting for sync.

Fix courtesy of @rjl493456442 : we now extend the init-check, so that detect that "yes, it's already initalized, and a sync is ongoing".

There's an additional fix also, which I ran into when doing a guerilla partial-wipe by keeping the ancient folder but deleting chaindata. It would not allow pruning the state histories back to zero.

[holiman]

I think this is not a sufficient fix. The code further down:

	// Load the meta objects in range [nhead+1, ohead]
	blobs, err := rawdb.ReadStateHistoryMetaList(freezer, nhead+1, ohead-nhead)
	if err != nil {
		return 0, err
	}
	batch := db.NewBatch()
	for _, blob := range blobs {
		var m meta
		if err := m.decode(blob); err != nil {
			return 0, err
		}
		rawdb.DeleteStateID(batch, m.root)
	}

The call to ReadStateHistoryMetaList will read the entire chunk of data into memory, since it internally does

	return db.AncientRange(stateHistoryMeta, start-1, count, 0)

using 0 as the maxbytes ( if maxBytes is not specified, 'count' items will be returned if they are present).

Also, I'm not sure how this would behave if we try to load 5M, but 0-4M are already gone.

A better fix might be to something like this:

	if nhead != 0 { // truncating to zero is always possible
		ohead, err = freezer.TruncateHead(0)
		if err != nil {
			return 0, err
		}
		return int(ohead - nhead), nil
	}

But if resetting to 0, we should also set back the tail to zero. Hm... @rjl493456442 thoughts?

[rjl493456442]

This fix is not correct.

The leftover state history can be in range of [tail, head].
The reset to 0 won't work because the state segment before tail is not longer accessible, but we try to load them here blobs, err := rawdb.ReadStateHistoryMetaList(freezer, nhead+1, ohead-nhead)

[holiman]

Closing in favour of #28718