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.
this will block the entire range to accept any new cmd which is not we wish to. Especially we need to wait respCache.CopyInfo which take long.

We need to optimize this part, e.g. will not create the actual range until the copy is done.