Add integration test for basic task processing

[markstory]

We should have an end-to-end test that covers basic task consumption and completion with a lightweight 'worker'.

• Clone TaskworkerClient into the taskbroker
• Add a fixed number of tasks to kafka
• start up taskbroker consumer/grpc
• Use one or more TaskworkerClient instances to fetch and 'complete' tasks.

We should be able to count the number of task activations that were created, fetched and completed. If the totals differ we've lost tasks, or double executed tasks.