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.
You should also verify the contents of the response cache for both ranges. Should be pretty easy -- use client cmd IDs for the puts and then look same up here.