trie: add start key to NodeIterator constructors

The 'step' method is split into two parts, 'peek' and 'push'. peek
returns the next state but doesn't make it current.

The end of iteration was previously tracked by setting 'trie' to nil.
End of iteration is now tracked using the 'iteratorEnd' error, which is
slightly cleaner and requires less code.

trie/secure_trie.go

@@ -156,8 +156,10 @@ func (t *SecureTrie) Root() []byte {
 	return t.trie.Root()
 }
 
-func (t *SecureTrie) NodeIterator() NodeIterator {
-	return t.trie.NodeIterator()
+// 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 {
+	return t.trie.NodeIterator(start)
 }
 
 // CommitTo writes all nodes and the secure hash pre-images to the given database.