wrong statistics of the region tree

[nolouch]

Bug Report

remove region may cause the wrong result.

pd/server/core/region_tree.go

Lines 141 to 159 in 51ad29c

	// remove removes a region if the region is in the tree.
	// It will do nothing if it cannot find the region or the found region
	// is not the same with the region.
	func (t *regionTree) remove(region *RegionInfo) {
	if t.length() == 0 {
	return
	}
	item := &regionItem{region: region}
	result := t.tree.Find(item)
	if result == nil || result.(*regionItem).region.GetID() != region.GetID() {
	return
	}
	t.totalSize -= region.approximateSize
	regionWriteBytesRate, regionWriteKeysRate := region.GetWriteRate()
	t.totalWriteBytesRate -= regionWriteBytesRate
	t.totalWriteKeysRate -= regionWriteKeysRate
	t.tree.Remove(result)
	}