tpcc: add -scatter option

[petermattis]

After restarting a cluster, leases for the tpcc tables can be concentrated on 1 or 2 nodes. It would be useful for faster iteration to add a -scatter option to scatter the replicas/leases across the nodes. See kv for related code which performs scatter operations after splitting, though I think the work here is even simpler as we can just scatter all of the ranges for the tpcc tables.