DV update are broken for updates on new field

A segmemnt written with Lucene70Codec failes if it ties to update
a DV field that didn't exist in the index before it was upgraded to
Lucene80Codec. We bake the DV format into the FieldInfo when it's used
the first time and therefor never go to the codec if we need to update.
yet on a field that didn't exist before and was added during an indexing
operation we have to consult the coded and get an exception.
This change fixes this issue and adds the relevant bwc tests.