journald: automatically rotate journal files when the clock jumps backwards

journald: automatically rotate journal files when the clock jumps backwards

As soon as we notice that the clock jumps backwards, rotate journal files. This
is beneficial, as this makes sure that the entries in journal files remain
strictly ordered internally, and thus the bisection algorithm applied on it is
not confused.

This should help avoiding borked wallclock-based bisection on journal files as
witnessed in #4278.