Correct tracking of checkpoint_memory_overhead

Discovered whilst investigating MB-38012 that we are currently tracking checkpoint memory overhead incorrectly. We appear to track the allocations of the checkpoint_index map, but not of the keys used in them.

Consider the case where we have a large disk checkpoint that is entirely non-resident in the checkpoint (i.e. everything has been expelled) then the keys are a substantial part of this. In the case of MB-38012 we are tracking approx 80 bytes overhead per item, but this does not include the keys which are ~48 bytes. We were tracking 27GB memory overhead in the checkpoint but in reality this is more like 40GB.