Refactor DropMessage timestamp model + new DropMessage serialization test

See #154.
1) In DropMessage objects, the timestamp is no longer represented by an integer Unix timestamp but by an actual Java Date-object.
2) When (de)serializing DropMessages, use 64-bit `long` as intermediate representation type, not 32-bit `int`
3) Have a new DropMessageSerialization test that lets a test expect proper JSON.
4) Compare the time stamp in the first existing DropMessageSerialization as well.