logalloc: segment_store, segment_pool: return const segment* from segment_from_idx() const
logalloc: segment_store, segment_pool: return const segment* from segment_from_idx() const

Maintain the const chain by returning a const segment*
from segment_from_idx() const overload.

And add a respective mutable overload to return a mutable segment*.

This is done for a similar change in idx_from_segment.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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