Add a pg_lsn data type, to represent an LSN.

doc/src/sgml/datatype.sgml

src/backend/utils/adt/Makefile

src/backend/utils/adt/pg_lsn.c

src/include/catalog/catversion.h

src/include/catalog/pg_operator.h

src/include/catalog/pg_proc.h

src/include/catalog/pg_type.h

src/include/fmgr.h

src/include/postgres.h

src/include/utils/pg_lsn.h

src/test/regress/expected/pg_lsn.out

src/test/regress/parallel_schedule

src/test/regress/serial_schedule

src/test/regress/sql/pg_lsn.sql