Enhancement

Under disagg arch, when handling a query, tiflash-compute will establish a snapshot for the query "TableScan" on the write node side. Then for each segment, tiflash-compute will call fetchDisaggPages to fetch the delta-layer data from write node to read node. Only after all segment are finish fetching from write node to read node, then the write node can graceful shutdown without leading to query failure.

All those disagg snapshot are managed by WNDisaggSnapshotManager. So when shutting down tiflash write node, tiflash should wait the snapshot manager to be empty before shutting down.