trie: ensure resolved nodes stay loaded

Commit 40cdcf1183 broke the optimisation which kept nodes resolved
during Get in the trie. The decoder assigned cache generation 0
unconditionally, causing resolved nodes to get flushed on Commit.

This commit fixes it and adds two tests.

trie/hasher.go

@@ -58,7 +58,7 @@ func (h *hasher) hash(n node, db DatabaseWriter, force bool) (node, node, error)
 			return hash, n, nil
 		}
 		if n.canUnload(h.cachegen, h.cachelimit) {
-			// Evict the node from cache. All of its subnodes will have a lower or equal
+			// Unload the node from cache. All of its subnodes will have a lower or equal
 			// cache generation number.
 			return hash, hash, nil
 		}