Fix: zKB fetch not fetching new kills

Fixed a bug with the negative time offset, causing zKB fetch to fetch the
same 200 kills all over again if there were more than 200 kills within the
offset timeframe.
Now a second fetch cycle is done until an empty result or a new result
gets returned (or the safety limit is hit).
Enhanced error logging for API import/IDFeed
Fix for external Character ID sanity check

There was a way to create a Pilot object with a valid external ID that
would return 0 as external ID when called getExternalID().