timesyncd: save clock to disk everytime we get an NTP fix, and bump clock at boot using this

This is useful to make sure the system clock stays monotonic even on
systems that lack an RTC.

Also, why we are at it, also use the systemd release time for bumping
the clock, since it's a slightly less bad than starting with jan 1st,
1970.

This also moves timesyncd into the early bootphase, in order to make
sure this initial bump is guaranteed to have finished by the time we
start real daemons which might write to the file systemd and thus
shouldn't leave 1970's timestamps all over the place...