Convert a DateTime to a string representation with millisecond accuracy

[SteveBush]

#2944 . In Sqlite implementation, the timestamp has only second precision. as it was using "s". Convert it to a dateime format with millisecond accuracy.

Note, I chose not to convert it here to UniversalTime(). Dispatch converts to universal time so maybe we should be consistent here.

DispatchedAt?.ToUniversalTime()

[SteveBush]

Note, you will need to merge in #2941 before the tests will succceed.

[iancooper]

As this changes the format of a datetime string, I will probably push this to V10 as we can document the change

[SteveBush]

As this changes the format of a datetime string, I will probably push this to V10 as we can document the change

Sounds good. To be specific, the consumer format of datetime does not change just the precision and format that is stored in the database internally. DateTime has an accuracy of 10ms to 30ms depending on OS and computer. Unless the consumer is looking at the database directly, they will only see a change in the accuracy of DateTime.