core/state: access trie through Database interface, track errors (#14589)

With this commit, core/state's access to the underlying key/value database is
mediated through an interface. Database errors are tracked in StateDB and
returned by CommitTo or the new Error method.

Motivation for this change: We can remove the light client's duplicated copy of
core/state. The light client now supports node iteration, so tracing and storage
enumeration can work with the light client (not implemented in this commit).

les/request_test.go

@@ -62,7 +62,7 @@ func tfCodeAccess(db ethdb.Database, bhash common.Hash, number uint64) light.Odr
 		return nil
 	}
 	sti := light.StateTrieID(header)
-	ci := light.StorageTrieID(sti, testContractAddr, common.Hash{})
+	ci := light.StorageTrieID(sti, crypto.Keccak256Hash(testContractAddr[:]), common.Hash{})
 	return &light.CodeRequest{Id: ci, Hash: crypto.Keccak256Hash(testContractCodeDeployed)}
 }