Add tests for checkpoint service

[gavin-norman-sociomantic]

The checkpoint service is a feature that needs to avoid regressions. There are several tests that can be added, from which some are very easy and some are more complicated to implement.

Easy:

• Write into the multiple channels and expect that all buckets appear in the checkpoint file exactly one time
• Confirm that the file doesn't exist on the clean exit of the node
• Start the node with the data and checkpoint file and expect buckets truncated on the right spots

Medium:

• Write into the channels distributed around for more than < "number of cached files" constant > buckets worth of data, so some buckets gets closed and confirm that they appear immidiately after opening the new channel, and after that expect that they are not found there

Hard:

• Open new buckets while the node is inside checkpoint dump cycle, so the checkpoints are scheduled to be dumped at the end of the cycle