cache should track memory overhead

Currently the cache only accounts for the cache values in the memtracker. Each key seems to have 88 or so bytes of memory usage as well (potentially rounded up due to allocation overhead, etc).

For the DRAM cache, this is still a ~700:1 ratio assuming 64kb block sizes, but for PMEM, where we expect hundreds of GBs of block cache, a 700:1 ratio may turn out to be somewhat substantial.