Commit 9a3cebea changed things so that parallel workers didn't obtain
any lock of their own on tables they access.  That was clearly a bad
idea, but I'd mistakenly supposed that it was the intended end result
of the series of patches for simplifying the executor's lock management.
Undo that change in relation_open(), and adjust ExecOpenScanRelation()
so that it gets the correct lock if inside a parallel worker.

In passing, clean up some more obsolete comments about when locks
are acquired.

Discussion: https://postgr.es/m/468c85d9-540e-66a2-1dde-fec2b741e688@lab.ntt.co.jp