Rename verbose ckpt to checkpoint, add some more messages to it.

The messages relate to the volume of data being written in checkpoints.

I found these helpful when debugging an issue that stalled throughput during checkpoints.

I was surprised we have 'ckpt' in the API - I know the abbreviation, but I don't think it's obvious to others. I'm happy to revert the name change if it's not worth the API change.