Restore sane locking behavior during parallel query.

Restore sane locking behavior during parallel query.

Commit 9a3cebeaa 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