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.
Let's not do this yet. I've been having some thoughts about needing to do everything that's part of a raft command as a single write batch, so will probably implement something more fundamental and transparent.