gpfdist uses the global xid & timestamp to distinguish whether each
connection belongs to the same external scan or not.

ORCA generates a unique scan number for each ExternalScan within the
same plan, but not accross plans. So, within a transaction, we may issue
multiple external scans that do not get differentiated properly,
producing different results.

This commit patches that by using a different scan number accross plans,
just like what planner does. Ideally gpfdist should also take into
account the command-id of the query to prevent this problem for other
cases such as prepared statements.