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.
Probably should change this to glog.Errorf. Also, print the error as part of the log statement using %v so we can better see what went wrong with the DropRange command.