lvconvert: reusing old cache pool metadata needs -Zn

lvconvert: reusing old cache pool metadata needs -Zn

When cache pool is reused for a new cached volume, there is
normally no need to 'keep' old cache-pool metadata as this
could cause major data lose.

Unlike with 'lvcreate -H -LX --cachepool' conversion, this lvconvert
path left the metadata unzeroed - partly for making easier some
debugging, but this was rather a bug.

So to keep possible reattach of 'unzeroed' metadata, user
now has to use 'lvconvert -Zn' for such conversion. In this case
the prompt will appear about possibe data loss and to proceed,
user has to confirm such operation. Without -Zn metadata are wiped.