For partitioned tables, in case of DELETE/UPDATE operation on the root
table, we must acquire ExclusiveLock on the root and leaf partitions, so
that any other operation requesting lock higher than AccessShareLock and
ShareLock must wait on QD.

If we don't acquire lock for the leaf partitions, other concurrent DML
operation may be dispatched to the segments and a deadlock can occur.
Refer to the test case added for GDD.

resultRelations may not have all the entries including the root and leaf
partition tables, so in this fix we explicitly acquire locks on all the
tables.

While GDD is on, it will be able to detect the deadlock and cancel one
of the query, so we don't acquire locks on the leaf parts if the DML
operation is on the root.
Co-authored-by: NAlexandra Wang <lewang@pivotal.io>
Signed-off-by: NAlexandra Wang <lewang@pivotal.io>
Co-authored-by: NAlexandra Wang <lewang@pivotal.io>