storage_service: handle_state_removing: restore_replica_count under _async_gate
storage_service: handle_state_removing: restore_replica_count under _async_gate

Track the background restore_replica_count fiber
so it be awaited on in stop() by closing the
_async_gate.

Fixes #10672

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

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