vinyl: don't track tuples that are already tracked in secondary index

When scanning a secondary index, we actually track each tuple in the
transaction manager twice - as a part of the interval read from the
secondary index and as a point in the primary index when retrieving
the full tuple. This bloats the read set - instead of storing just one
interval for a range request, we also store each tuple returned by it,
which may count to thousands. There's no point in this extra tracking,
because whenever we change a tuple in the primary index, we also update
it in all secondary indexes. So let's remove it to save us some memory
and cpu cycles.

This is an alternative fix for #2534
It should also mitigate #3197