Move pg_upgrade TODO to TODO wiki.

contrib/pg_upgrade/TODO

-o  remove copy_dir code, or use it
-o  handle large object comments;  this is difficult to do because the large object
-   doesn't exist when --schema-only is loaded
-o  consider using pg_depend for checking object usage in version.c
-o  if reindex is necessary, allow it to be done in parallel with pg_dump custom format
-o  migrate pg_statistic by dumping it out as a flat file, so analyze is not necessary