[IValue] initialize timestamp with min_data_time not not_a_date_time

When comparing not_a_data_time with "now", not_a_date_time would be "greater".
This allows easier comparasion; not_a_date_time would only be an additional
hint that the data is invalid, but there's a dedicated flag for this information.
[ISputnikCommand] Call Notify() also if data becomes invalid.

As the validity flag is part of the data, observers should now this state change too.
[DBWriter] Eliminate IValue-copy lastloggedvalue and use timestamp information to determine
if something has changed.
[DBWriter] Reformatting and documentation.
[DBWriter] removed unused variable
[DBWriter] on REMOVE_ALL do not unsubscribe to the capas required to bootstrap again.
[DBWriter] Also create columns using selectors, using TEXT as datatype.

Also reminding the user to change the type if desired.
[DBWriter] Implement getter/setter for logevery.

The var will be made private the next commit...
[DBWriter] made logevery private and using the getter where needed.