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?
It will be not hard to track the count for each range...thus we just need one iterating to get exact split key.