allocation_strategy: Hide migrators
allocation_strategy: Hide migrators

After IMR rework the only lsa-migrating functionality is standard one
that calls move constructors on lsa slots. Hide the whole thing inside
allocation-strategy.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla