storage_service: add memory limiter semaphore getter
storage_service: add memory limiter semaphore getter

The memory limiter semaphore is going to be useful for limiting
alternator memory as well, so it's hereby exposed via a getter.

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