vinyl: ignore unknown .run, .index and .vylog keys

Currently, if we encounter an unknown key while parsing a .run, .index,
or .vylog file we raise an error. As a result, if we add a new key to
either of those entities, we will break forward compatibility although
there's actually no reason for that. To avoid that, let's silently
ignore unknown keys, as we do in case of xrow header keys.