The new setting should allow to control query complexity on leaf nodes
excluding the final merging stage on the root-node. For example, distributed
query that reads 1k rows from 5 shards will breach the `max_rows_to_read=5000`,
while effectively every shard reads only 1k rows. With setting `max_rows_to_read_leaf=1500`
this limit won't be reached and query will succeed since every shard reads
not more that ~1k rows.