The intranode shard balancing loop only stopped when the most-loaded
and least-loaded shard were the same (src == dst), meaning it would
keep issuing migrations until the load difference reached exactly 0.
This caused unnecessary migrations for negligible imbalances.
Apply the same is_balanced() threshold check that is already used for
inter-node balancing, so that intranode migrations stop when the
relative load difference between shards is within the configured
size_based_balance_threshold (default 1%).
(cherry picked from commit aaead10e5d)
8a71a5ba88