Cursor dropping triggering too aggressively

Cursor dropping appears to trigger too aggressively for the checkpoint memory condition (see MB-29227).

Adding additional debug logging, we see that the calculation for cursor_dropping_checkpoint_mem_upper_mark bytes value is incorrect - e.g.

261:2019-04-24T12:22:16.042548+01:00 INFO (default) Triggering cursor dropping as checkpoint_memory (388 MB) exceeds cursor_dropping_checkpoint_mem_upper_mark (50%, 0 MB). Attempting to free 877 MB of memory.

This is attempting to trigger cursor dropping if the checkpoint memory (388 MB) exceeds 50% of the bucket_quota, however it is incorrectly calculating this limit as 0 MB. As such it is always triggering cursor dropping - and moreover is attempting to free more memory than necessary.