Rename pg_sequence_read_tuple() to pg_get_sequence_data()

Rename pg_sequence_read_tuple() to pg_get_sequence_data()

This commit removes log_cnt from the tuple returned by the SQL function.
This field is an internal counter that tracks when a WAL record should
be generated for a sequence, and it is reset each time the sequence is
restored or recovered.  It is not necessary to rebuild the sequence DDL
commands for pg_dump and pg_upgrade where this function is used.  The
field can still be queried with a scan of the "table" created
under-the-hood for a sequence.

Issue noticed while hacking on a feature that can rely on this new
function rather than pg_sequence_last_value(), aimed at making sequence
computation more easily pluggable.

Bump catalog version.

Reviewed-by: Nathan Bossart
Discussion: https://postgr.es/m/Zsvka3r-y2ZoXAdH@paquier.xyz