The LDAP role manager's _cache_pruner fiber used
invoke_on_all() to reload permissions on every shard.
Since auth::service::start() runs on all shards in
parallel via invoke_on_all(), the pruner on shard X
could call reload_all_permissions() on shard Y before
shard Y finished start() and set its permission loader,
hitting SCYLLA_ASSERT(_permission_loader). The same
cross-shard race existed during shutdown.
Each shard runs its own pruner instance, so reloading
locally is sufficient — all shards are still covered.
This also removes redundant N-squared reload calls.
Refs SCYLLADB-1679
f75e5ac65b