Add replay code to Hyperleveldb port. #1106

@michaelcahill Here's a first pass at implementing the ReplayIterator code for the Hyperleveldb port.  Let's discuss it next time we talk as I may have taken a few liberties or made some assumptions along the way.  While it builds cleanly, I didn't see any obvious test code, so it is not yet ready to merge.
Is that what HyperLevelDB does?  I'm curious because there's still no guarantee that the backup is completely up-to-date if there are concurrent updates, so I wonder why this is necessary...