allocation_strategy: expose preferred allocation size limit
allocation_strategy: expose preferred allocation size limit

Our premier allocation_strategy, lsa, prefers to limit allocations below
a tenth of the segment size so they can be moved around; larger allocations
are pinned and can cause memory fragmentation.

Provide an API so that objects can query for this preferred size limit.

For now, lsa is not updated to expose its own limit; this will be done
after the full stack is updated to make use of the limit, or intermediate
steps will not work correctly.

kjniemi/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,duarten/scylla,duarten/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla