Handle rescan of RemoteQuery node correctly

Handle rescan of RemoteQuery node correctly

We never had this support and we never felt the need because the use of FQS was
limited for utility statements and simple queries which can be completed
pushed down to the remote node. But in PG 10, we're seeing errors while using
cursors for queries which are FQSed. So instead of forcing regular remote
subplan on such queries, we are adding support for rescan of RemoteQuery node.

Patch by Senhu <[email protected]>