Fix assorted bugs related to identity column in partitioned tables

Fix assorted bugs related to identity column in partitioned tables

When changing the data type of a column of a partitioned table, craft
the ALTER SEQUENCE command only once.  Partitions do not have identity
sequences of their own and thus do not need a ALTER SEQUENCE command
for each partition.

Fix getIdentitySequence() to fetch the identity sequence associated
with the top-level partitioned table when a Relation of a partition is
passed to it.  While doing so, translate the attribute number of the
partition into the attribute number of the partitioned table.

Author: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Dmitry Dolgov <9erthalion6@gmail.com>
Discussion: https://www.postgresql.org/message-id/3b8a9dc1-bbc7-0ef5-6863-c432afac7d59@gmail.com