mpt: modify refcounted storage scheme to make GC possible

Add "active" flag into the node data and make the remainder modal, for active
nodes it's a reference counter, for inactive ones the deactivation height is
stored.

Technically, refcounted chains storing just one trie don't need a flag, but
it's a bit simpler this way.

pkg/core/statesync/module_test.go

@@ -24,7 +24,7 @@ func TestModule_PR2019_discussion_r689629704(t *testing.T) {
 	require.NoError(t, tr.Put([]byte{0x06, 0x03}, []byte("leaf4")))
 
 	sr := tr.StateRoot()
-	tr.Flush()
+	tr.Flush(0)
 
 	// Keep MPT nodes in a map in order not to repeat them. We'll use `nodes` map to ask
 	// state sync module to restore the nodes.