run ALTER TABLE EXPAND PARTITION PREPARE in separate transactions (#13152)
run ALTER TABLE EXPAND PARTITION PREPARE in separate transactions (#13152)

For the current stage-1 implementation in gpexpand, we run ALTER TABLE EXPAND PARTITION
PREPARE in a single transaction with the database as the dimension, then when there are many
partition tables in a database, the ALTER TABLE statement will hold a large number of locks,
which is likely to cause shared memory exhaustion.

Therefore, we split it into multiple transactions to execute separately, this does not break existing
processes.
greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb