Can't unmarshal a partitioned cache

[chlowell]

Easy to repro with a unit test in partitioned_storage_test.go:

func TestMarshal(t *testing.T) {
	mgr := newPartitionedManagerForTest(nil)
	b, err := mgr.Marshal()
	if err != nil {
		t.Fatal(err)
	}
	err = mgr.Unmarshal(b)
	if err != nil {
		t.Fatal(err)
	}
}

Unmarshal returns the error Unmarshal(*storage.InMemoryContract) only supports structs that have the field AdditionalFields or implements json.Unmarshaler. And that's spot on, storage.InMemoryContract doesn't have an AdditionalFields field.

[bgavrilMS]

Solved by #456

[chlowell]

Reopening because #456 didn't merge and it's still impossible to unmarshal a partitioned cache