Add support for point in time queries

From Matt Williams at Rackspace:
I used to be an Oracle DBA when I worked for the army and although I tend to loath Oracle for many reasons, one of the reasons I enjoyed it was the ability to do a "flashback" query where I would set a database to a point in time before executing a statement. Obviously, this came at a price and retention was based on how much space you wanted to dedicate to the copy-on-write operations required to do this, but space is cheap and I used the feature quite often!

From Stewart:
"This would be trivial to implement for the ARCHIVE storage engine

But if you want fancy things like UPDATEs and DELETEs along with indexes, you're probably not exclusively using ARCHIVE

In our transactional engines (InnoDB/HailDB and PBXT) this would seem quite non-trivial to implement. The way these engines store previous row versions does not lead themselves to go too far back in time in reasonable computational time (for queries on current and old versions)."