BUG/MINOR: clock: automatically adjust the internal clock with the boot time

BUG/MINOR: clock: automatically adjust the internal clock with the boot time

This is a better and more general solution to the problem described in
this commit:

    BUG/MINOR: checks: postpone the startup of health checks by the boot time

Now we're updating the now_offset that is used to compute now_ms at the
few points where we update the ready date during boot. This ensures that
now_ms while being stable during all the boot process will be correct
and will start with the boot value right after the boot is finished. As
such the patch above is rolled back (we don't want to count the boot
time twice).

This must not be backported because it relies on the more flexible clock
architecture in 2.8.