Checkpoints are always purged if there are no cursors in them

We are aggressively removing checkpoints from memory. In this case I loaded 100k items into Couchbase, waited, and observed that there was only one item in each checkpoint manager. We should keep checkpoints in memory if we have space.

Mikes-MacBook-Pro:ep-engine mikewied$ management/cbstats 10.5.2.34:12000 checkpoint | grep num_checkpoint_items | cut -c 44- | awk '{s+=$1} END {print s}'

1024