compute split key for range splitting

Added a method to compute a split key based on the size. To do this, I had to look at raw keys from the engine and decide whether they are an MVCC metadata key or a timestamped value and if so, find the metadata key. To do all of this, I had to expose the methods mvcc{En,De}Code and was forced to do the binary decoding/encoding of keys manually, which leaks MVCC implementation details.
@jiangmingyang Can you suggest an interface to improve on this?
I had looked at that before, but mvcc.Scan takes a (user-space, i.e. not binary) key and I'm scanning the range without MVCC for efficiency (and in order to not mingle with it more than I have to).
Really all I need is some facility that when given some (binary, i.e straight from the engine) key, will give me the associated key name or tell me it's not MVCC (right now there's a panic).