If a partition range is not present locally,
`partition_ranges_owned_by_this_shard` assigns it to shard 0, which can
overload shard 0. To address this, this commit adds a `shard_id_hint`
to the mapreduce request. When `shard_id_hint` is set, the entire
partition range in the request is handled by the specified shard.
The `shard_id_hint` is set by the new tablet-aware mapreduce algorithm,
introduced in `dispatch_to_tablets`. This algorithm balances the
workload across shards, so the changes in this commit ensure that
load balancing is preserved, even during events such as tablet splits.
Fixes: scylladb#21831
26403df9ea