Introduced by #1751.
Steps to reproduce:

1. Load a TPC-H dataset into TiFlash
2. Restart the TiFlash process and don't do read / write on those tables
3. Check the store size reported to PD, or "Store size" in the Grafana panel

Previously, we will restore the store size by calling DeltaMergeStore::restoreStableFiles in DelatMergeStore constructor. But now the store objects are Lazily initialized and not able to report the real store size.
This will make PD confused and cause lots of rebalancing between TiFlash nodes.
https://github.com/pingcap/tics/blob/65209cd0f7ffd330e9c5261231a64f2aa66cc28c/dbms/src/Storages/DeltaMerge/DeltaMergeStore.cpp#L1905-L1918