lvmetad: pvscan: do not scan and read ignored MDAs from PVs
Metadata areas which are marked as ignored should not be scanned
and read during pvscan --cache. Otherwise, this can cause lvmetad
to cache out-of-date metadata in case other PVs with fresh metadata
are missing by chance.
Make this to work like in non-lvmetad case where the behaviour would
be the same as if the PV was orphan (in case we have no other PVs
with valid non-ignored metadata areas).