core: do not spawn jobs or touch other units during coldplugging

Because the order of coldplugging is not defined, we can reference a
not-yet-coldplugged unit and read its state while it has not yet been
set to a meaningful value.

This way, already active units may get started again.

We fix this by deferring such actions until all units have been at
least somehow coldplugged.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=88401