gluon-autoupdater: add version_file option to uci settings
autoupdater: convert to Lua

In addition, a new config field 'version_file' is added to remove the last
Gluon-specific part from the autoupdater package.
autoupdater: add a utility function to parse RFC3339 timestamps
autoupdater: implement new probablity logic
gluon-autoupdater: adjust for new updater logic

The probability field is removed, updates are performed once per night between
04:00 and 05:00.
autoupdater: add -o (oneshot) switch

This switch is used for the fallback mode which ensures that updates are
performed even when a node is off when updates are usually done.
gluon-autoupdater: add locking to ensure no two instances of the autoupdater run in parallel
gluon-autoupdater: unconditionally perform an update 5 minutes after boot when the whole PRIORITY delay has passed
autoupdater: add new fields to the sample manifest
autoupdater: all fallback for situations with no valid time available