#1915 fix retrieving the historical value of a thing with at-historical-timestamp

[thjaeckle]

There were several issues with the logic:

• the first issue was that as fallback when no at-historical-revision was provided, the "Latest" sequence number was selected -> which resulted in returning the latest state
  • that was fixed by not falling back at that place to this "fallback value", but work with a Optional instead
• the second problem was that for a thing which was freshly created (and as a result of that "freshness" no Snapshot was yet written into the snapshot store), Ditto did not distinguish correctly between "has not yet a snapshot" and "is not existing at the provided timestamp"

Both issues are fixed by this PR and using at-historical-timestamp should now work.

Fixes: #1915

[thjaeckle]

@alstanchev @kalinkostashki could you please have a look?

[alstanchev]

LGTM 👍