core/state: enable partial-functional reader (snapshot integration pt 3) (#30650)

It's a pull request based on https://github.com/ethereum/go-ethereum/pull/30643

In this pull request, the partial functional state reader is enabled if **legacy snapshot
is not enabled**. The tracked flat states in pathdb will be used to serve the state
retrievals, as the second implementation to fasten the state access.

This pull request should be a noop change in normal cases.

core/state/database.go

@@ -179,10 +179,19 @@ func (db *CachingDB) Reader(stateRoot common.Hash) (Reader, error) {
 	// is optional and may be partially useful if it's not fully
 	// generated.
 	if db.snap != nil {
+		// If standalone state snapshot is available (hash scheme),
+		// then construct the legacy snap reader.
 		snap := db.snap.Snapshot(stateRoot)
 		if snap != nil {
 			readers = append(readers, newFlatReader(snap))
 		}
+	} else {
+		// If standalone state snapshot is not available, try to construct
+		// the state reader with database.
+		reader, err := db.triedb.StateReader(stateRoot)
+		if err == nil {
+			readers = append(readers, newFlatReader(reader)) // state reader is optional
+		}
 	}
 	// Set up the trie reader, which is expected to always be available
 	// as the gatekeeper unless the state is corrupted.