Speed improvement for large object restore.

src/bin/pg_dump/pg_backup_archiver.c

src/bin/pg_dump/pg_backup_archiver.h

src/bin/pg_dump/pg_backup_custom.c

src/bin/pg_dump/pg_backup_files.c

src/bin/pg_dump/pg_backup_null.c

src/bin/pg_dump/pg_backup_tar.c