Merge commit '21fef0309c2383e66b67976d4e00a0447693b37e' into tiflash-…

…hot-write-sche

# Conflicts:
#	server/core/store.go

server/core/store.go

@@ -33,6 +33,7 @@ const (
 	gb                     = 1 << 30 // 1GB size
 	initialMaxRegionCounts = 30      // exclude storage Threshold Filter when region less than 30
 	initialMinSpace        = 1 << 33 // 2^33=8GB
+	slowStoreThreshold     = 80
 
 	// EngineKey is the label key used to indicate engine.
 	EngineKey = "engine"