Refactor ItemMetaData class

The ItemMetaData class presents an issue in it's current for because we cannot convert relative expiration times in a good way. The current solution is somewhat of a hack. We just do the conversion in the ep_engine.cc file after we have decoded the meta data. With the Item class we have an itemAllocate function that takes care of creating an item and makes sure the expiration time is correct. The ItemMetaData class should have an API similar to this.