hints: invalidate when pvscan --cache sees a new PV

hints: invalidate when pvscan --cache sees a new PV

An idea from Zdenek for better ensuring valid hints by invalidating
them when pvscan --cache <device> sees a new PV, which is a case
where we know that hints should be invalidated.  This is triggered
from systemd/udev logic, and there may be some cases where it would
invalidate hints that the existing methods wouldn't detect.