Make StockItem qty columns deferred by default

We had been loading used,sold,remaining,firstsale,lastsale each time we
loaded any StockItem.  Most of the time this isn't necessary!  Defer
these by default and mark them as group "qtys" so they can be undeferred
in any query using options(undefer_group('qtys')).
Speed up "stock level check" function

Undefer the StockType.instock column.  Also move the query to where it's
used.