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?
Why not check here whether or not key is a metadata key or versioned value key and only add a single sample for all of the values for a particular key, with the byte count equal to the sum of all versions + metadata?