Fetching a thing at a non-existing historical revision (e.g. in future) results in status 503

[thjaeckle]

Example request:

curl 'https://ditto.eclipseprojects.io/api/2/things/foo:bar?fields=thingId,_revision,attributes&at-historical-revision=785'

Response after 15 seconds:

{"status":503,"error":"ask.error","message":"There was an internal timeout.","description":"Please retry the performed action in order to improve resiliency."}

And even worse than that - during the 15 seconds, all other requests to that Thing will fail, as all access is prevented as part of the at-historical-revision calculation.