Hold _async_gate in all REST-facing async operations so that stop()
drains in-flight requests before teardown, preventing use-after-free
crashes when REST calls race with shutdown.
A centralized gated() wrapper in set_storage_service (api/storage_service.cc)
automatically holds the gate for every REST handler registered there,
so new handlers get shutdown-safety by default.
run_with_api_lock_internal and run_with_no_api_lock hold _async_gate on
shard 0 as well, because REST requests arriving on any shard are forwarded
there for execution.
Methods that previously self-forwarded to shard 0 (mark_excluded,
prepare_for_tablets_migration, set_node_intended_storage_mode,
get_tablets_migration_status, finalize_tablets_migration) now assert
this_shard_id() == 0. Their REST handlers call them via
run_with_no_api_lock, which performs the shard-0 hop and gate hold
centrally.
Fixes: SCYLLADB-1415
6a91d046f3