Ignore 'idling' annotations whose timestamp is earlier than the existing 'idling' annotation

[leonardr]

If an 'idling' annotation comes in for a book and its timestamp shows it was generated earlier than the existing 'idling' annotation, the existing annotation should be preserved and the attempt to overwrite it rejected.

If the client is bug-free this can only happen when you are reading the same book simultaneously on two different devices, but as is I believe this can happen as devices come in and out of network connectivity.