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?
The incoming request timestamp needs to be updated so we write intents with the correct timestamp. The outgoing response header timestamp is updated in order to communicate back to the client how to increase it's final commit timestamp.