Allow asynchronous execution in more cases.

In commit 27e1f1456, create_append_plan() only allowed the subplan
created from a given subpath to be executed asynchronously when it was
an async-capable ForeignPath.  To extend coverage, this patch handles
cases when the given subpath includes some other Path types as well that
can be omitted in the plan processing, such as a ProjectionPath directly
atop an async-capable ForeignPath, allowing asynchronous execution in
partitioned-scan/partitioned-join queries with non-Var tlist expressions
and more UNION queries.

Andrey Lepikhov and Etsuro Fujita, reviewed by Alexander Pyhalov and
Zhihong Yu.

Discussion: https://postgr.es/m/659c37a8-3e71-0ff2-394c-f04428c76f08%40postgrespro.ru

src/backend/nodes/readfuncs.c

@@ -2194,6 +2194,7 @@ _readSubqueryScan(void)
 	ReadCommonScan(&local_node->scan);
 
 	READ_NODE_FIELD(subplan);
+	READ_ENUM_FIELD(scanstatus, SubqueryScanStatus);
 
 	READ_DONE();
 }