The row cache memory footprint changed after switch to B+
because we no longer have a sole cache_entry allocation, but
also the bplus::data and bplus::node. Knowing their sizes
helps analyzing the footprint changes.
Also print the size of memtable_entry that's now also stored
in B+'s data.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>