RPKI: ask a cache without data yet again soon

A validator that is still running its first validation answers a query
with No Data Available (error code 2). That ended the session, and the
next attempt came after the retry interval: 600 seconds when the cache
had not yet said otherwise, which it cannot have before it has data.
For those ten minutes after a validator started, nothing was validated.

No Data Available is now asked again after 30 seconds, or after
retry_seconds if that is shorter, and logged as information rather than
as a warning. Other failures wait as before, after trying the next
cache if there is one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/content/docs/guides/rpki.md

@@ -64,6 +64,11 @@ rpki {
 When the data expires, Ferrix discards it, and no route is considered
 invalid until new data is received.
 
+A validator that has just started reports that it has no data available
+until its first validation run is complete, which takes several minutes.
+Ferrix then asks again after 30 seconds, or after `retry_seconds` if that
+is shorter, rather than after the retry interval.
+
 ### Running a Validator
 
 The following command starts Routinator with an RTR server on port 3323: