Fix failure in WHERE CURRENT OF after rewinding the referenced cursor.

In a case where we have multiple relation-scan nodes in a cursor plan,
such as a scan of an inheritance tree, it's possible to fetch from a
given scan node, then rewind the cursor and fetch some row from an
earlier scan node.  In such a case, execCurrent.c mistakenly thought
that the later scan node was still active, because ExecReScan hadn't
done anything to make it look not-active.  We'd get some sort of
failure in the case of a SeqScan node, because the node's scan tuple
slot would be pointing at a HeapTuple whose t_self gets reset to
invalid by heapam.c.  But it seems possible that for other relation
scan node types we'd actually return a valid tuple TID to the caller,
resulting in updating or deleting a tuple that shouldn't have been
considered current.  To fix, forcibly clear the ScanTupleSlot in
ExecScanReScan.

Another issue here, which seems only latent at the moment but could
easily become a live bug in future, is that rewinding a cursor does
not necessarily lead to *immediately* applying ExecReScan to every
scan-level node in the plan tree.  Upper-level nodes will think that
they can postpone that call if their child node is already marked
with chgParam flags.  I don't see a way for that to happen today in
a plan tree that's simple enough for execCurrent.c's search_plan_tree
to understand, but that's one heck of a fragile assumption.  So, add
some logic in search_plan_tree to detect chgParam flags being set on
nodes that it descended to/through, and assume that that means we
should consider lower scan nodes to be logically reset even if their
ReScan call hasn't actually happened yet.

Per bug #15395 from Matvey Arye.  This has been broken for a long time,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/153764171023.14986.280404050547008575@wrigleys.postgresql.org

src/backend/executor/execScan.c

@@ -263,6 +263,12 @@ ExecScanReScan(ScanState *node)
 {
 	EState	   *estate = node->ps.state;
 
+	/*
+	 * We must clear the scan tuple so that observers (e.g., execCurrent.c)
+	 * can tell that this plan node is not positioned on a tuple.
+	 */
+	ExecClearTuple(node->ss_ScanTupleSlot);
+
 	/* Rescan EvalPlanQual tuple if we're inside an EvalPlanQual recheck */
 	if (estate->es_epqScanDone != NULL)
 	{