Direct runner should enforce encodability of unbounded source checkpoints

As far as I can tell, https://github.com/apache/incubator-beam/blob/master/runners/direct-java/src/main/java/org/apache/beam/runners/direct/UnboundedReadEvaluatorFactory.java currently uses the reader's getCheckpoint() only as an in-memory object, i.e. it's not exercising that 1) the checkpoint can be encoded at all, and 2) that the reader can be resumed from an encoded/decoded checkpoint.

I've seen cases in code reviews where people implemented a non-serializable checkpoint, and direct runner tests didn't catch that because of this issue.