Use Date type for date strings

[smcvb]

Feature Description

Replace the date String fields for actual Date types.
Important in this process is to ensure the created sorting index (based on the event timestamp and sequence number) keeps a similar performance as it currently has.
Furthermore, we should make sure this adjustment keeps the current ordering based on timestamp and seqNo in working order.

Current Behaviour

Currently, the Mongo Extension uses Strings to contain date times.

Wanted Behaviour

The Mongo Extension should use the Date type to refer to date fields.