storage: stats-based rebalancing can't handle large numbers of splits and scatters

Discovered in #17644. Here's a RESTORE with stats-based rebalancing enabled:

![screenshot 2017-08-15 13 20 53](https://user-images.githubusercontent.com/882976/29328138-854d6e9e-81bf-11e7-9f49-2dbe7d54329f.png)

And without:

![screenshot 2017-08-15 12 50 10](https://user-images.githubusercontent.com/882976/29328127-78343364-81bf-11e7-870c-a4dcd1055311.png)
