vinyl: force read view in iterator in autocommit mode

Every iteration over a secondary index tracks a point in the transaction
manager (due to lookup in the primary index). As a result, if the user
calls 'select' or 'pairs' over a huge data set, it will consume a lot of
memory due to this tracked points, even if the user doesn't uses
transactions.

To mitigate this, let's send all read only transactions to read view
immediately so that tracking is disabled completely during iteration.
Note, with this patch select() called outside a transaction doesn't
populate the cache any more, but it seems to be OK as caching large
select() requests results in cache thrashing.

Closes #2534