Add "existing" populators for subqueryload

Add "existing" populators for subqueryload

Fixed bug in subquery loading where an object encountered as an
"existing" row, e.g. already loaded from a different path in the
same query, would not invoke subquery loaders for unloaded attributes
that specified this loading.  This issue is in the same area
as that of :ticket:`3431`, :ticket:`3811` which involved
similar issues with joined loading.

Change-Id: If111a76b0812010905b0ac811276a816779d297f
Fixes: #3854