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).

trie/secure_trie.go

@@ -156,6 +156,11 @@ func (t *SecureTrie) Root() []byte {
 	return t.trie.Root()
 }
 
+func (t *SecureTrie) Copy() *SecureTrie {
+	cpy := *t
+	return &cpy
+}
+
 // NodeIterator returns an iterator that returns nodes of the underlying trie. Iteration
 // starts at the key after the given start key.
 func (t *SecureTrie) NodeIterator(start []byte) NodeIterator {