LSA memory reclaimer logic assumes that the amount of memory used by LSA
equals: segments_in_use * segment_size. However, LSA is also responsible
for eviction of large objects which do not affect the used segmentcount,
e.g. region with no used segments may still use a lot of memory for
large objects. The solution is to switch from measuring memory in used
segments to used bytes count that includes also large objects.
Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>