Improve performance of POCO Date-types

[ewoutkramer]

The POCO date types have their ObjectValue set to string, and will parse the string into a DateTimeOffset every time a something is "done" with the type (e.g. comparing, getting Utc dates, etc). This causes performance issues in the CQL engine, so we should try to cache the conversion from string to DateTimeOffset (invalidating it when the ObjectValue/Value is changed).