When LSA reclaimer cannot reclaim more space by compaction, it
will reclaim data by evicting from evictable regions.
Currently the only evictable region is the one owned by the row cache.
Requiring alignment means that there must be 64K of contiguous space
to allocate each 32K segment. When memory is fragmented, we may fail
to allocate such segment, even though there's plenty of free space.
This especially hurts forward progress of compaction, which frees
segments randomly and relies on the fact that freeing a segment will
make it available to the next segment request.
