Restore relmapper state early enough in parallel workers.

Restore relmapper state early enough in parallel workers.

We need to do RestoreRelationMap before loading catalog-derived
state, else the worker may end up with catalog relcache entries
containing stale relfilenode data.  Move up RestoreReindexState
too, on the principle that that should also happen before we
do much of any catalog access.

I think ideally these things would happen even before InitPostgres,
but there are various problems standing in the way of that, notably
that the relmapper thinks "active" mappings should be discarded at
transaction end.  The implication of this is that InitPostgres and
RestoreLibraryState will see the same catalog state as an independent
backend would see, which is probably fine; at least, it's been like
that all along.

Per report from Justin Pryzby.  There is a case to be made that
this should be back-ed.  But given the lack of complaints
before 6e086fa2e and the short amount of time remaining before
17.0 wraps, I'll just put it in HEAD for now.

Discussion: https://postgr.es/m/ZuoU_8EbSTE14o1U@pryzbyj2023