KVS Migrations Will Always Hotspot The Transactions Table

[jeremyk-91]

The technique of writing everything to the destination KVS as a single transaction leads to hotspotting in the new world. Even _transactions2 (which we don't use anyway as we blow away the coordination store) won't help, because it is one timestamp.

Solutions probably need to both write values at multiple timestamps (though all less than the fast-forward timestamp) and use _transactions2.