Relay storage node doesn't exec local object operations, so it doesn't need
shard configuration.
Add `required` bool parameter to `engineconfig.IterateShards`. Make it to
panic if it is `true`, and immediately return otherwise. Pass `false` if
node is configured as relay in app (it also prevents panic).
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Add `shard_pool_size` config to `storage` section. Set app default to 20.
Pass the value to `WithShardPoolSize` option.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
