Refactor the SrcItem object so that information about subqueries is stored

Refactor the SrcItem object so that information about subqueries is stored
in a separately allocated Subquery object.  This reduces the memory requirements
for SrcItem and makes the code run faster.  It also provides an expansion path
for subquery processing that does not burden simple queries.  The current
checking mostly works, but there are still issues that need to be tracked
down and fixed.

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