Improve Asserts checking relation matching in parallel scans.

Improve Asserts checking relation matching in parallel scans.

table_beginscan_parallel and index_beginscan_parallel contain
Asserts checking that the relation a worker will use in
a parallel scan is the same one the leader intended.  However,
they were checking for relation OID match, which was not strong
enough to detect the mismatch problem fixed in 126ec0bc7.
What would be strong enough is to compare relfilenodes instead.
Arguably, that's a saner definition anyway, since a scan surely
operates on a physical relation not a logical one.  Hence,
store and compare RelFileLocators not relation OIDs.  Also
ensure that index_beginscan_parallel checks the index identity
not just the table identity.

Discussion: https://postgr.es/m/2127254.1726789524@sss.pgh.pa.us