Improve performance of Poco Instant

[marcovisserFurore]

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).

See for inspiration PR #2529 where it is already implemented for FhirDateTime.

[ewoutkramer]

The existing Instant datatype is already stored as a DateTimeOffset internally, so we do not have to cache or speed things up for this datatype. Closing this issue.