Copy the INSERT query in postgres_fdw

 Copy the INSERT query in postgres_fdw
 
 When executing the INSERT with batching, we may need to rebuild the
 query when the batch size changes, in which case we pfree the current
 string. We must not release the original string, stored in fdw_private,
 because that may be needed in EXPLAIN ANALYZE. So make copy of the SQL,
 but only for INSERT queries.
 
 Reported-by: Pavel Stehule
 Discussion: https://postgr.es/m/CAFj8pRCL_Rjw-MCR6J7VX9OF7MR6PA5K8qUbrMvprW_e-aHkfQ%40mail.gmail.com