vinyl: use point lookup explicitly where appropriate

We never use vy_point_lookup directly, instead we open vy_read_iterator,
which automatically falls back on vy_point_lookup if looking for exact
match (EQ + full key). Due to this we can't add a new point lookup
specific argument (we would have to propagate it through the read
iterator, which is ugly). Let's call vy_point_lookup directly when we
know that vy_read_iterator will fall back on it anyway.