don't update the serial number if there are no changes
don't update the serial number if there are no changes

Note that this now means it's possible for the RTR database to have only one serial number that appears to be expired but is actually still valid, if the cache of the global RPKI goes unchanged for more than RETENTION_HOURS.

rhansen/rpstir,rhansen/rpstir,rhansen/rpstir,rhansen/rpstir,rhansen/rpstir