Fix tuple routing to initialize batching only for inserts

 Fix tuple routing to initialize batching only for inserts
 
 A cross-partition update on a partitioned table is implemented as a
 delete followed by an insert. With foreign partitions, this was however
 causing issues, because the FDW and core may disagree on when to enable
 batching.  postgres_fdw was only allowing batching for plain inserts
 (CMD_INSERT) while core was trying to batch the insert component of the
 cross-partition update.  Fix by restricting core to apply batching only
 to plain CMD_INSERT queries.
 
 It's possible to allow batching for cross-partition updates, but that
 will require more extensive changes, so better to leave that for a
 separate patch.
 
 Author: Amit Langote
 Reviewed-by: Tomas Vondra, Takayuki Tsunakawa
 Discussion: https://postgr.es/m/20200628151002.7x5laxwpgvkyiu3q@development