postgres_fdw: Remove duplicate code in DML execution callback functions.
postgres_fdw: Remove duplicate code in DML execution callback functions.

postgresExecForeignInsert(), postgresExecForeignUpdate(), and
postgresExecForeignDelete() are coded almost identically, except that
postgresExecForeignInsert() does not need CTID.  Extract that code into
a separate function and use it in all the three function implementations.

Author: Ashutosh Bapat
Reviewed-By: Michael Paquier
Discussion: https://postgr.es/m/CAFjFpRcz8yoY7cBTYofcrCLwjaDeCcGKyTUivUbRiA57y3v-bw%40mail.gmail.com

50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb