autoupdater: rename oneshot updates to fallback, improve handling for frequent runs

The oneshot update now won't be considered until 24h after the priority delay
has passed (so regular updates should have at least one chance to do the updates
even for PRIORITY=0 updates)
gluon-autoupdater: perform fallback updates hourly
Merge pull request #52 from freifunk-gluon/autoupdater-fixes

Autoupdater fixes