Enhancement

When an MPP task encounters a lock during region reads, or when the region is not available locally, TiFlash will fall back to remote read using the coprocessor via client-c. In this case, it would be preferable to use the local TiFlash store itself as the remote read source. This has several advantages:

1. Reduces network transmission overhead.
2. Coprocessor requests sent through client-c are load-balanced across TiFlash nodes. If the request happens to be routed to a TiFlash node that is down or restarting, it can introduce unnecessary latency — in extreme cases, this may significantly increase query latency.