Add tests for checkpointing

[kkrugler]

We need to verify that the DomainDB and UrlDB states are checkpointed/savepointed properly.

For checkpointing, we need a test that enables checkpointing (in memory), causes the job to fail, and the verifies that we don't replay everything that we've processed (so checkpointing worked).

For savepointing, I think we need a test that stops a job, then restarts with a different parallelism, and verifies that state is re-distributed properly.

[kkrugler]

See https://ci.apache.org/projects/flink/flink-docs-release-1.4/dev/stream/testing.html, and the "Testing checkpointing and state handling" section.

[kkrugler]

See also the test_resume_savepoint script in the Flink code.