Resource cannot be well limited in large transactions

## Enhancement
For large transactions (e.g. DELETE ... affecting ~40k rows), TiDB splits the SQL into hundreds or thousands of KV requests and dispatches them concurrently (default prewrite concurrency is 128). These batch requests can instantly saturate TiKV resources (coprocessor, scheduler CPU), creating traffic spikes that consume large goroutines and lock resources, delaying or blocking other transactions. Timeouts and retries further amplify the spikes, leading to lock conflict storms and high overhead from failed transactions.