This mini-series adds an _async_gate to storage_service that is closed on stop()
and it performs restore_replica_count under this gate so it can be orderly waited on in stop()
Fixes#10672Closes#10922
* github.com:scylladb/scylla:
storage_service: handle_state_removing: restore_replica_count under _async_gate
storage_service: add async_gate for background work