add InternalSplit command & tests

* added a method CopyInto to the response cache, req'd for initializing a new range during a split.
* added an InternalSplit command. Making the split happen without a transitory period in which things can go wrong is subtle so this needs a good review and likely some changes.
* added some basic tests, but the transition should be bulletproofed using more tests eventually.
See my earlier command about the algorithm for split. The range leader directs the distributed txn which includes the meta1/meta2 updates. We can't have that done by every range replica separately!