Add support for date and time ConvertedTypes

Initial proposal for date, time, finetime, timestamp, timestamptz and interval types.

Look forward to others thoughts.  

For date: days since epoch keeps us small to minimize bytes per value but it also has issues in that it isn't a common format.  

On time/finetime, I've never been a big fan of these with timezone since it is somewhat unclear what that even means.  As such, I specified without.